Use upstream SDL2

[sharkwouter]

Upstream SDL2 is currently in a mostly usable state for the PSP, even if it's not quite on par with our fork yet. We should make PRs to upstream to get it to the point where it's at least as good and ship it instead of our fork. This will reduce work for us and make sure SDL2 for PSP stays in a good spot.

Before we do this, here is a list of changes needed in upstream SDL:

• [x] Fix sdlmain (PR: https://github.com/libsdl-org/SDL/pull/5076)
• [x] Add cmake support (PR: https://github.com/libsdl-org/SDL/pull/5074)
• [x] Make rendering large textures work (PR: https://github.com/libsdl-org/SDL/pull/5080)
• [x] Make the game controller API work on PSP (PR: https://github.com/libsdl-org/SDL/pull/5079)
• [x] Use lightweight mutex for improving performance (PR: https://github.com/libsdl-org/SDL/pull/5106)
• [x] Improve performance when vsync is enabled (PR: https://github.com/libsdl-org/SDL/pull/5104)
• [x] Use the correct numbers for thread priorities in audio (PR: https://github.com/libsdl-org/SDL/pull/5112)
• [x] Improve audio support (PR: https://github.com/libsdl-org/SDL/pull/5105)
• [x] Make rendering to texture work (PR: https://github.com/libsdl-org/SDL/pull/5129)
• [x] Fix artifacting when textures are changed (PR: https://github.com/libsdl-org/SDL/pull/5107)
• [x] Make blending/transparency work consistently when the blend mode is set to SDL_BLENDMODE_BLEND (PR: https://github.com/libsdl-org/SDL/pull/5129)
• [x] Add filesystem support (PR: https://github.com/libsdl-org/SDL/pull/5158)

[sharkwouter]

Here we need to make sure the quality of the commits is good, changes are as small as needed and PRs are specific to one issue/improvement. We want the end result to be maintainable and we want upstream to be able to accept our changes.

We also don't want to repeat loading a small amount of textures causing out of memory errors because the vram is full.

[sharkwouter]

I tested the old commit for rendering to texture and it's not really a good implementation. It works if there is only 1 texture being rendered to. If there are more layers, it doesn't work anymore.

[sharkwouter]

Most of the rendering issues we face have been at least partially fixed in this PR to one of the SDL2 forks for PSP: https://github.com/joel16/SDL2/pull/5

[sharkwouter]

Upstream SDL is now used in the toolchain.