With the new image editor drawing there were was some mutual exclusive
functionality. When rendering the alpha was shown correctly or the pure
emissive colors were shown correctly, but never both. The cause of this
is that the image_gpu did not used the correct alpha mode when generating
gpu textures for non-images (render results, compositors viewer)

The implementation always checked the alpha_mode. Alpha mode is an
attribute for images, but aren't set for non images. This patch adds
a more detailed check to ensure that the gpu texture is premultiplied.

The issue has been tested using several bug report files and production
files.