Fixes for Legacy GL/GLES

[complexlogic]

#1296 was originally targeting Modern OpenGL, but when the decision was made later to lower support to GL/GLES 2.0, a couple of things were missed. These won't be an issue for the 99% of us that aren't playing the game on 20 year old hardware, but since we did make the promise in #1296 to keep support for those devices, these issues should be addressed.

The first commit requires the extension GL_OES_element_index_uint for OpenGL ES 2.0 devices, because the spec otherwise doesn't support element buffers with 32 bit unsigned int. The only reason I wanted to support GLES 2 in #1296 was to maintain compatibility for Raspberry Pi 3 and older devices, and they support this extension, so this won't be an issue. Otherwise, modern embedded GPUs such as those on Raspberry Pi 4-5 support GLES 3 which doesn't require any extensions for this feature.

The second commit switches the glyph loading to use GL_LUMINANCE texture format for GL 2, and GLES 2.0 (unless the GL_EXT_texture_rg extension is available), instead of GL_RED which is unavailable on those versions.

[dgruss]

We could support GLES2-compatible indices (see #1381) without the extension, but it makes the code much more complicated. I'm not convinced it is worth it. Maybe we should just centrally adjust what the exact compatibility claims are?

[dgruss]

Could we simplify the GLES2 path to always use GL_LUMINANCE? It is available in core GLES2, and the text shader samples the red component, so the optional GL_EXT_texture_rg check does not appear necessary for correctness. GLES3/GL3 can continue using GL_RED.

Also, we could define only the needed local constant (GL_LUMINANCE = $1909) in URenderer_OpenGL.pas, keeping the change smaller without avoid exposing many unrelated deprecated symbols.

[dgruss]

Looks good to me