Improve texure painting 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 GPU textures used the same texture space
and needed to be uploaded 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.
**TODO**
This patch is still work in progress.
When drawing on a tiled image in the image editor the partial updates draws scaled down.
Could be related that the tiled runtime isn't replicated.