gdkgl: Remove flipping when downloading GL texture

gdkgl: Remove flipping when downloading GL texture

This fix is correct and fixes:

  1) GL textures being upside down in the inspector. They are getting
     downloaded because they've been created in a different GL context
  2) GL textures being upside down in the cairo renderer (same reason)

However, it breaks the testsuite. We do the flipping via the projection
matrix, but most of the shaders don't care about that.