sdl3: Add OpenGL and Vulkan contexts

[RobLoach]

Adds the SDL3 OpenGL and Vulkan contexts so that the SDL3 video driver can load hardware-rendered cores. Also fixes up a few things within the video driver itself:

• Can remember window positions
• Consolidates shared code across the driver and contexts into sdl3_common
• Fixes a rendering bug when applying the scissor rect
• Addresses windowed fullscreen vs normal fullscreen

Test

1. Compile, and run RetroArch
2. Set the video driver to "sdl3"
3. Load up all three of the libretro-video samples
4. Watch the logs, when loading
5. Expect to see that it shows the SDL3 context driver in the logs

[cscd98]

Does this include GLES?