Previously the alpha component of the test texture data was always set
to 255 and the data was read back as RGB so that the alpha component
is ignored. Now the alpha component is set to a generated value and
the data is read back a second time as RGBA to verify that Cogl is not
doing any premult conversions when the internal texture and target
data is the same.
http://bugzilla.clutter-project.org/show_bug.cgi?id=2414
This verifies that calling cogl_texture_get_data returns the same data
uploaded to the texture. The bottom quarter of the texture is replaced
using cogl_texture_set_region. It tries creating the texture with
different sizes and flags in the hope that it will hit different
texture backends.
