topic/snacman#22 shader reloading

[Adnn]

closes #22, closes #54

• Provide an ImGui button to explicitly recompile all programs.
• Avoid a deadlock with a dirty workaround.
• Move Freetype instance up into main, to outlive RenderThread.
• Use lockguard instead of manual lock/unlock.
• Match ImguiUi operations with RAII guards.
• Make shader recompilation a blocking call in main thread (via synchro).
• Handle shader recompilation error without crashing.
• Add DearImgui to snac-viewer.
• [snac-viewer] Implement half-viewport hot shader recompilation.
• [snac-viewer] Handle shader recompilation error without crashing.
• Factorize technique's program recompilation, fix snacman compilation.
• Fix error, where program is not "unbound" after use.
• Rename effect loading functions from program-file.
• Store parameters in Camera class, provide as uniforms.
• Enable opengl synchronous debug.
• Start implementing shadows, show the depth map.
• Implement effect file loading, use a custom depth pass.
• Implement GUI control of the render mode.
• Handle multiple meshes in snac-viewer, used to draw floor.
• Use single matrix for light viewpoint, modeled as a Camera class.
• Sample shadow with sampler2d(trivial approach, with shadow acne).
• Use WarningRepository also for textures.
• Use a shadow sampler instead of explicit conditional test.
• Implement controllable shadow bias.
• Add controllable depth-map filtering.
• Move shadow rendering mode to a separate DrawerShadow class.
• DrawerShadows cache GL resources instead of re-creating them each frame.
• Set normals on rectangle, giving floor specular + diffuse.
• Implement shader macro definition, used to consolidate phong shaders.
• clean-up a debug output of preprocessed shader source.
• Replace the plain map repositories with a RepositoryStack class.
• Rework the Renderer API, making it more modular, and consume it in Pass.
• Rename Renderer classes.
• Fix a resource name in snacman.
• Revert to blocking call to renderBackend(), defer the button action.
• Move shader's gamma correction to a separate include file.
• Upgrade internal dependencies.