Splitting out GL and Trade functionality to separate libraries

[mosra]

This is the first huge potentially breaking thing on the path to Vulkan support. Basically everything from the Magnum namespace that had anything to do with OpenGL is now moved into a new Magnum::GL library. That goes similarly for include paths and CMake imported targets. But don't worry too much (just a little), this is done with the utmost care to preserve as much backwards compatibility as possible.

I'm doing this one little step at a time to avoid getting crazy. Things to do, in order:

• Separate Trade library out of Magnum (mostly just buildsystem changes, resulting in a new Magnum::Trade CMake imported target (and library)
• Move everything GL-related to Magnum/GL subdir and a new library (at this step this means even the Image class and everything, which means also the whole Trade library and all plugins now depend on GL library), not adding a new namespace yet
• Add a new Magnum::GL namespace
  • Convert Magnum::Extensions::GL::* to Magnum::GL::Extensions::* ?
  • Convert Platform::Context to Platform::GLContext
• Provide deprecated compatibility headers (e.g. <Magnum/Buffer.h> that points to <Magnum/GL/Buffer.h>)
• Provide deprecated aliases (e.g. Magnum::Buffer that is a typedef to Magnum::GL::Buffer)
• Adapt rest of the project to not use deprecated headers and aliases (again, until this is done, the non-deprecated build will fail)
• Adapt Developers Guide to use new file locations and namespaces
• Adapt all docs to not use deprecated headers and aliases (Doxygen won't complain because of the compat versions)
  • Adapt all @section identifiers to use the GL namespace
  • Rename @extension to @gl_extension
• Extract PixelFormat / PixelType enums back to root namespace and provide some mapping to GL types (need to check what Vulkan has here)
• Separate GL-specific stuff out of PixelStorage and put the rest back to root namespace
  • Temporary assertions for pixel storage things that are not supported in ES / WebGL (will be later replaced with proper implementation coming from the Vulkan part)
• Put Image classes back to root namespace
  • also tests
• Separate MeshPrimitive and Mesh::IndexType enum out of GL lib
• Think about how to separate sampler state out of GL lib (also enums straight in Trade, how to map to GL?) It's a bunch of Sampler enums in the root namespace now
• Now that Trade does not depend on GL anymore, fix the linking and fix that also Primitives and all plugins
• Verify that all dependent repos and plugins and examples build with the new lib w/o any source change
• Update all packages and CIs to explicitly use WITH_GL and WITH_TRADE no need to, this is enabled by default
• Introduce TARGET_GL option, allowing to enable/disable GL support in other libraries
• Make the SDL2 and GLFW app usable with TARGET_GL being turned off (i.e., so it doesn't even link to GL)
• Look in the OpenGL / platform overview docs and check that nothing is terribly wrong
  • Avoid saying that GL is the "core part", it's not
  • Adapt magnum-gl-info "screenshot"
• Rename magnum-info to magnum-gl-info. Backwards compatibility for this is not considered, please complain if you need that.
• Rename all MAGNUM_ASSERT_*_SUPPORTED() macros to MAGNUM_ASSERT_GL_*_SUPPORTED() or MAGNUM_GL_*() (how to handle backwards compat?)
  • also MAGNUM_VERIFY_NO_ERROR()
• Changelog entries for all of above

After all the above this PR could be merged back to master, I think. Separating the remaining libraries from GL will be done incrementally over time.

Things I'm not sure how to do:

• Backward compatibility so people linking to Magnum will be linked to Magnum::GL and Magnum::Trade as well -- there would be a cyclic dependency with this, so nope
• Should GL::BufferImage have GL::PixelFormat or Magnum::PixelFormat? The former would require a mapping table back to Magnum::PixelFormat, the latter will feel alien (formatExtra etc.) It's the GL-specific format now.

Cc: @Squareys

[codecov-io]

Codecov Report

Merging #233 into master will increase coverage by 0.78%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##           master     #233      +/-   ##
==========================================
+ Coverage   45.84%   46.62%   +0.78%     
==========================================
  Files         345      355      +10     
  Lines       15261    15539     +278     
==========================================
+ Hits         6996     7245     +249     
- Misses       8265     8294      +29

Impacted Files	Coverage Δ
.../DebugTools/Implementation/AbstractBoxRenderer.cpp	0% <ø> (ø)	⬆️
src/Magnum/GL/Attribute.h	69.23% <ø> (ø)
src/Magnum/Audio/Renderer.h	100% <ø> (ø)	⬆️
src/Magnum/Audio/Context.h	100% <ø> (ø)	⬆️
src/Magnum/GL/Implementation/State.h	0% <ø> (ø)
src/Magnum/GL/BufferImage.h	5.47% <ø> (ø)
src/Magnum/GL/AbstractQuery.cpp	5.76% <ø> (ø)
src/Magnum/GL/BufferImage.cpp	6.34% <ø> (ø)
src/Magnum/DebugTools/ForceRenderer.h	0% <ø> (ø)	⬆️
...agnum/GL/Implementation/TransformFeedbackState.cpp	0% <ø> (ø)
... and 215 more

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[mosra]

Trade library split was merged to master in 920db7a.

[mosra]

Expecting angry people with pitchforks and torches doing a witch hunt on me now, haha 🙈 🔥

Documentation and proper release notes / upgrading guide coming soon with the 2018.04 release.