Changes to viewport state to resolve texture paint color
selection introduced a side effect wherein the correct
attachment size of a framebuffer was reset. This size is
needed when scissor regions are disabled to return the
state to its correct default. When this default was wrong,
certain operators would have incorrect offsets.
To maintain consistency with the OpenGL backend, the
Metal backend independently tracks the raw attachment
size using default_width/height. This will also reset to zero
when attachments are all removed, unlike other state which
may be retained.
Authored by Apple: Michael Parkin-White
Jason Fielder
Michael Parkin-White