This extension is effectively a backport of GLES3's internalformat
handling to GLES 1/2.  It guarantees that sized internalformats specified
for textures and renderbuffers have at least the specified size stored.
That's a pretty minimal requirement, so I think it can be dummy_true and
exposed as a standard in Mesa.

As a side effect, it also allows GL_RGB565 to be specified as a texture
format, not just as a renderbuffer.  Mesa had previously been allowing 565
textures, which angered DEQP in the absence of this extension being
exposed.

v2: Allow 2101010rev with sized internalformats even on GLES3, citing the
    extension spec.  Extend extension checks for GLES2 contexts exposing
    with texture_float, texture_half_float, and texture_rg.
v3: Fix ALPHA/LUMINANCE/LUMINANCE_ALPHA error checking (GLES3 CTS
    failures)
v4: Mark GL_RGB10 non-color-renderable on ES, fix A/L/LA errors on GLES2
    with float formats.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>