Remove ES3 option, restrict use to WebGL2

[flibitijibibo]

ES3 was, for a long time, the only choice for a lot of places:

• Xbox
• Switch
• Stadia
• iOS/tvOS
• Emscripten
• FNADroid

Nowadays, thanks to FNA3D, it now looks like this...

• Xbox D3D11 (UWP), GLon12 (GDK)
• Switch Vulkan
• Stadia Vulkan
• iOS/tvOS MoltenVK
• Emscripten
• FNADroid Never supported, even then would be Vulkan-only

We're so close to making ES3 unnecessary, sadly Emscripten is completely screwed thanks to WebGPU. But, that doesn't mean we necessarily have to keep all of ES3, we could hypothetically just restrict it to WebGL2 use instead.

The checklist:

• Vulkan by default
• Remove FORCE_ES3 environment variable
• Rename useES3 to webGL, update SDL_GetPlatform checks
• See what WebGL2 looks like, maybe we can strip down the useES3 paths

If somebody makes a WebGPU-based ICD for Vulkan we can just strip all the ES3 code out.