Blender Git Loki

Asset: Dropping Material assets on material slot under mouse cursor.

This patch allows dropping material assets from material slot under the mouse
cursor. Before this change the material slot had to be hand-picked from the
properties panel.

For consistency it is chosen to do this in any shading mode as the tooltip shows
what is exactly going to happen during release.

The feature also works for other object types than Meshes as it uses the drawn surface on the
GPU to detect the material slots. Performance of this patch has been tested with AMD GCN3.0
cards and are very responsive.

Reviewed By: fclem, Severin

Differential Revision: https://developer.blender.org/D12190