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.
This includes using `''` instead of `None` in some cases where an empty
string conveys 'nothing' equally well as `None`; in such cases keeping the
type the same rather than switching to another type is preferred.
The `file_loading` function is called deferred by asyncio, and can thus
be called when the operator has already stopped loading. This is fixed by
not referring to `self` in that function, and taking the logger from the
outer scope.
