Support drag-and-drop of savestate files.

Most likely you want to load the state, not see an error message.
Support render-to-offset within a framebuffer.

Fixes #6324.
Merge pull request #6325 from unknownbrackets/ui-tweaks

Support drag-and-drop of savestate files
Merge pull request #6326 from unknownbrackets/gpu-minor

Support render-to-offset within a framebuffer