Test that GL_TEXTURE_COMPRESSED_IMAGE_SIZE behaves consistently on cubemap.
Test that GL_TEXTURE_COMPRESSED_IMAGE_SIZE behaves consistently on cubemap.

It returns different value based on whether the cube map is immutable
(allocated using setStorage()) or a sequence of setImage() calls, well,
at least on my NVidia. That should not happen, workaround incoming.

MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum,MiUishadow/magnum