Copy the format conversion code from GdkMemoryTexture
so we can produce all formats, and test them all.
The upload fast paths assume that the stride is a
multiple of four, so some of the padding values cause
it to fail. Apart from that, things seem to work for
all combinations.
Create textures with various characteristics (alpha, premultiplication,
stride) that trigger different code paths in the gl texture upload
function, and show the resulting images. If all goes well, they all
should look the same.
On my system, this tests texture upload for memory formats
GDK_MEMORY_B8G8R8A8_PREMULTIPLIED, GDK_MEMORY_R8G8B8A8, and
GDK_MEMORY_R8G8B8, and it works with both gl and gles.
