Blender Git Loki

Fix T82770: Artifacts when painting on generated transparent image

Regression introduced by {b17cca6966}. When centralizing the gpu texture
premultiplication setting it was assumed that generated images
(`IMA_TYPE_UV_TEST`) were stored as premultiplied. That assumption was
totally wrong as the alpha association is determined by the existing of
the float/byte buffer.

NOTE: This change will render generated images with pure emissive
colors (show colors when alpha=0.0) what might add more reports. Any
reports could be merged in the next report {T82790}.

Reviewed By: Cl�ment Foucault, Philipp Oeser

Differential Revision: https://developer.blender.org/D9585