mesa: Add helpers to extract GL_COLOR_INDEX to RGBA float/ubyte

mesa: Add helpers to extract GL_COLOR_INDEX to RGBA float/ubyte

We only use _mesa_make_temp_ubyte_image in texstore.c to convert
GL_COLOR_INDEX to RGBA, but this helper does more stuff than this.
All uses of this helper can be replaced with calls to
_mesa_format_convert except for this GL_COLOR_INDEX conversion.

This patch extracts the GL_COLOR_INDEX to RGBA logic to a separate
helper so we can use that instead from texstore.c.

In future patches we will replace all remaining calls to
_mesa_make_temp_ubyte_image in the repository (related to compressed
formats) with calls to _mesa_format_convert so we can remove
_mesa_make_temp_ubyte_image and related functions.

v2:
- Remove ‘for’ loop initial declaration. They are only allowed in C99 or C11
mode.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>