Improve texture painting/uv editing performance when limited scale is active.
Cause of the slow down is that the image editor draws the image in maximum resolution,
but the 3d viewport uses the limited scale. The variation reuses the same GPU texture
and needed to be uploaded/scaled twice to the GPU.

This patch will adds texture slots that can hold the scaled down and the maximum
resolution image. This would allow better cache hits and reuse of existing caches.

Maximum resolution textures are reused for limited scale when they fit to reduce memory
and CPU footprint.