Bootstrapping Vulkan support

[mosra]

This currently depends on #233, so I'm setting base branch to that instead of master so the changes are visible clearer. I'm also working on both at the same time. #233 is merged now.

Things to do:

• Initial library bootstrap (WITH_VK)
• Find module updates
  • Require CMake 3.7 not needed
  • Require C++14 for dependencies of this lib rather not, unless really needed by some feature
• TARGET_VK variable
• @fn_vk etc Doxygen aliases, update Coding Style for them
• Header generation using flextGL
  • some types are parsed with trailing whitespace, remove that
  • the global enums have a different order every time they are generated
  • the vk.xml file is not sorted in type dependency order, submit patch upstream (or hotfix in flextGL?)
  • don't export the flextVkInitInstance() / flextVkInitDevice() function (done just because there was no Instance/Device wrapper yet)
  • avoid global function pointers, fill a struct with them instead
  • add another flextVkInitDevice() for populating device-dependent pointers into another struct
  • clean up and submit the patches upstream (Vulkan support flextgl#15)
  • update the Developers Guide with generating Vulkan headers
• ShaderTools library -- 13dfb64
  • New AbstractShaderConverter plugin interface
  • Integrate glslang for GLSL validation and GLSL->SPIR-V compilation -- mosra/magnum-plugins@66a03a4
  • Integrate SPIRV-Tools (dis)assembler -- mosra/magnum-plugins@6b0680f
  • Integrate SPIRV-Tools linker for putting multiple stages together in a single file (heh, i thought glslang/shaderc did that, but no) postponed
  • Integrate SPIRV-Tools optimizer for SPIR-V optimization
  • Integrate SPIRV-Tools validator -- mosra/magnum-plugins@308df5b
  • others? glsl-optimizer, shaderc, glsl-validator(?), DirectXShaderCompiler) postponed
  • GLSL minifier (whitespace and comment removal, at least), this could be used in Magnum itself to help trimming down the size of the Shaders library postponed
  • Introspection for SPIR-V (querying attribute, uniform, ... names and bindings of an imported shader bytecode)
  • Specialization constants, specialization constant folding (because apparently even for a disabled branch all uniforms and inputs need to exist, sigh)
• Initial Instance wrapper
• Device property queries
  • Ability to disable even device extensions that don't need enabling (such as VK_KHR_driver_properties so we can the codepath for drivers that don't have it) postponed
• Initial Device wrapper
• Device selection using --magnum-device
• Enabling validation layers etc using --magnum-gpu-validation postponed
• Queue setup
  • Convenient queue retrieval using "out" params in addQueues()
• Command pool creation, command buffer allocation
  • Command buffer submit, reset postponed
• Memory allocation, mapping,
  • Flushing postponed
  • Usage docs
  • Dedicated strong type for "memory type bits" as it's otherwise too easy to mix up with size when allocating postponed
  • Support KHR_dedicated_allocation since it makes sense for large data and render targets (and we don't have anything else now anyway) postponed
  • Generic API for hooking up non-trivial memory allocators like the AMD one postponed, needs a whole new abstraction on top
• Image wrapper
  • Usage docs
  • (implicitly convertible) image view wrapper, ideally as simple as GL texture / framebuffer wrappers for the common case postponed
  • "Preset" tags for linear mappable / framebuffer images? postponed
• Buffer wrapper
  • Usage docs
  • Buffer view wrapper postponed
• Pipeline + shader setup postponed
• Vulkan-enabled alternative of the GL MeshTools::compile() postponed
• KHR_surface for Linux, Windows, Android and macOS postponed
• Initial magnum-vk-info utility
• Initial Vulkan function / limit mapping page
• Initial Vulkan extension support page
• Initial Vulkan wrapping docs, focusing on interoperability with raw code
• Vulkan-specific projection matrix creation. Would need to be moved out from Math::Matrix[34]::*Projection() to some API-specific header. postponed
• Initial base class for Vk tests, BUILD_VK_TESTS option
  • Decide what to set up in the Vulkan tester and what to leave up to the tests (will probably become clearer once we implement a few)
  • Setting up debug layers? Leak detection? later maybe
  • Setting up queues? later maybe
  • Ability to override the chosen device on command-line? -- yes, already done
• Test w/ SwiftShader on the CIs
  • Make a Windows, Mac and Linux build in magnum-ci
  • Make it working on Windows as well -- currently it fails with VK_ERROR_INITIALIZATION_FAILED (does it require some stuff being added to registry? ugh)
• make our own FindVulkan.cmake that finds just the library but correctly
  • Make it work with MoltenVK (libMoltenVk instead of libvulkan)
  • Make it work on MinGW (to avoid -DVulkan_LIBRARY=${MINGW_PREFIX}/lib/libvulkan.dll.a .. maybe it just needs to explicitly list libvulkan in addition to vulkan?
• Building on Travis CI against a minimal stub (can't be bothered to build some random driver there just yet)
  • Why is it looking for OpenGL even though everything else is disabled? -- not anymore since 89d9883
• Building on AppVeyor the same way -- d578d4d
• Integrating an initial Vulkan Triangle example -- mosra/magnum-examples@41c78ef
  • Gradually rewrite it using the new, less error-prone and less verbose wrappers postponed
• Basic translation of Magnum enums to raw Vulkan enums (will get deprecated/removed later once we have our own types there) -- b0a1719
• Conversion of basic Vulkan types into equivalent Magnum types (vectors, ranges, colors...) -- 3281a27

Cc: @Squareys

[mosra]

Now that the initial design got pretty stable, the overhead from branch switching and rebasing overweighted the advantages of being able to experiment, so I'm merging. Future development will happen directly on the master branch.