Add a test for various texture uploads

Add a test for various texture uploads

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.