mesa: Add GL_RED and GL_RG to ES3 effective internal format mapping.

mesa: Add GL_RED and GL_RG to ES3 effective internal format mapping.

The dEQP-GLES3.functional.fbo.completeness.renderable.texture.
{color0,depth,stencil}.{red,rg}_unsigned_byte tests appear to expect
GL_RED/GL_RG and GL_UNSIGNED_BYTE to map to GL_R8/GL_RG8, rather than
returning an INVALID_OPERATION error.

This makes perfect sense.  However, RED and RG are strangely missing
from the ES 3.0/3.1/3.2 spec's "Effective internal format corresponding
to external format and type" tables.  It may be worth filing a spec bug.

Fixes the 6 dEQP tests mentioned above.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>