Replace SDL_gpu with bgfx #276
Description
Motivation
We've noticed that SDL_gpu doesn't fit our needs in many situations: it has a strange transformation matrix model (which leads to rendering bugs at pixel boundaries on certain devices, as well as to reduced performance, and not only on older devices), is difficult to use with custom shaders (important for shadows, masking, cap insets etc.), and has docs that are difficult to use / are incomplete.
SDL itself is also a dependency we'd like to remove (we're only really keeping it around for SDL_gpu).
Using bgfx would give us much more flexibility over our rendering, allowing us to implement shadows, capInset images, 8bit textures (see #275), and more. And would also be a nicer toolkit to work with, given its greater user base, better docs, and clearer render pathway. It appears that bgfx is widely used on Android as well, and also seems to work with WebGL, which was an initial draw card for SDL.
Disadvantages
SDL_gpuallows us to render rounded corners out of the box. We're hardly making use of this feature though because its rendering is highly aliased. We'd prefer to go for a solution using cap inset images or similar anyway.- We'd have to rewrite the
UIScreenrender functions to a certain degree. This is probably unavoidable, but also gives us the opportunity to improve our rendering performance by batching more draw calls together. - If we removed
SDLwe'd also have to rethink our text rendering:SDL_ttfunsurprisingly relies onSDL. But this would not be necessary if we just replaceSDL_gpuwithbgfxinitially.