meta: Detect currently-unsupported integer CopyTexSubImage and complain.

meta: Detect currently-unsupported integer CopyTexSubImage and complain.

This code is unprepared for handling integer (particularly, the
baseFormat of the TexFormat comes out as GL_RGBA, not GL_RGBA_INTEGER,
so the direct call of Driver.ReadPixels crashes due to the int vs
non-int error checking not having happened).  I'm frankly tempted to
convert this code to MapRenderbuffer/MapTexImage rather than doing it
as meta ops, now that we have that support.

Improves the remaining crash in Intel oglconform for int-textures to
just a rendering failure.

NOTE: This is a candidate for the 8.0 branch.

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