mesa: don't use memcpy fast path for GetTexImage if base format is different

mesa: don't use memcpy fast path for GetTexImage if base format is different

The Mesa format can be RGBA8888_REV, the format/type can be
GL_RGBA/GL_UNSIGNED_BYTE, but the actual texture internal format can be
LUMINANCE_ALPHA, INTENSITY, etc. Therefore we should look at the base
internal format as well.

NOTE: This is a candidate for the stable branches.

Reviewed-by: Brian Paul <brianp@vmware.com>