imgui support for bespoke interfaces

[Robadob]

Mostly early development and testing now.

Goal is to provide support for #92
May eventually lead to deprecating internal FreeType support.

Current Progress

• CMake dependency integration
  • Could improve the generated CMakeLists.txt by adding explicit dependency on SDL2
  • Could replace CMakeLists.txt glob with specific files (in future once know what we need)
• Setup an interactable test UI pane
  • Make ImGui mouse input not break when FullScreen is enabled
• Display user specified UI
• Feedback updated env properties to model
• Validate properties passed to PanelVis.
• Read-only env prop support.
• Array env props?
• Prevent race conditions?
• Multi UI?
• F2 to hide all UIs
• Use native imgui support for all types rather than mapping
• Sort out widths etc
• Move debug menu to imgui (this kinda fixes where Text wrapping causes multi-line text to jiggle #99 was occuring)
• Allow users to specify how many dp for floating-point types
• Allow a button checkbox for int types (although would map to 0/1)
• Seperator
• Comments
• Documentation
• Update debug ui random seed if changed via HostAPI Rename rng seed, to initial rng seed
• Pyflamegpu
• Test pyflamegpu (especially env property element)
• Only allow each env prop to be in UI once (having multiple elements with same name causes input to break a bit)
• Multi window no overlap?
• Collapsible sections begin open?

[Robadob]

cc @ptheywood Mostly just review the CMake. Yeah it uses glob, but the files will only change on a configure (unless a user manually git checkouts or adds files), aka when GLOB runs. So doesn't bother me. Still more ideal than, having to keep track of if ImGui changes required files.

[mondus]

Design of this seems sensible and as discussed in the issue. It would be much easier to review with an example of usage within some of the model examples (given ped sits separately).

[Robadob]

It would be much easier to review with an example of usage within some of the model examples (given ped sits separately).

Circles is too simple (and quickly breaks down if you mess with forces). Less familiar with suitability of other models.

I will add a commit with this UI for all the boids examples, however they only demonstrate a small part of the functionality. Whereas the complexity of the ped model allows most to be used.