When using texture painting Blender could freeze, crash or not showing
any results depending on the platform being used.
There were several issues:
- texture painting could send a layer offset of -1, which would select
an incorrect layer
- source data wasn't copied correctly in the staging buffer as it
ignored `texture_unpack_row_length`
Pull Request: https://projects.blender.org/blender/blender/pulls/130973
492830e9ac