Keith Packard authored Jul 02, 2015 and Emma Anholt committed Jul 06, 2015

Fixes regressions since Eric's "don't make an FBO for the glyph atlas"
change.  The a1 upload was a fallback, as expected.  However, fallback
reads use glReadPixels() because there's no glGetTexSubImage2D() to
match glTexSubImage2D().  We were just binding the 0 FBO value, so the
glReadPixels() would throw a GL error instead of getting any data.
After the fallback was done we'd write back the undefined data to the
atlas, blowing away the entire rest of the atlas because we didn't
specify any bounds on our prepare.

To fix the fallbacks to actually work, we'd need a prepare path that
allocates some memory memory do a full glGetTexImage() into, then
memcpy out of that.  Instead, just dodge the general fallback by
implementing the specific upload we need to do here, which should also
be *much* faster at uploading a1 glyphs since it's not
readpixels/texsubimaging back and forth.

v3: Use CopyPlane to a temp pixmap for the upload
v4: Rewrite anholt's commit message to be from keithp's perspective
    (changes by anholt)

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>