Check Weights Installed Fix

[NullSenseStudio]

Due to the way the HeadlessDreamTexture operator is run it will not call its invoke() method. That could be fixed by calling bpy.ops.shade.dream_texture_headless() with 'INVOKE_DEFAULT' as its first argument, but that causes a whole stack trace to be reported instead of just its intended simple message for some reason. Same thing would happen if checking that the weights were installed within execute() method.

Instead it'll have to check for them in the original DreamTexture operator's invoke() method, and added an extra check within the render pass since that directly uses HeadlessDreamTexture.

Fixes #289