The `Image.gl_load()` call was changed in Blender commit
7ad802cf3ae500bc72863b6dba0f28a488fce3d1; the two parameters we were using
were removed.
This commit fixes the exception and makes the texture browser usable again,
but doesn't properly fix everything. The textures are drawn in the wrong
colour space, which will be fixed in another commit once I know how.
We shouldn't call any `gpu` functions in background mode. Since the texture
browser will never run when Blender is in background mode anyway, we can
simply assign `None` instead.
