Don't set / reset unscaled screen area when rendering to texture
Also fix a pre-10844 bug where scaling down a cockpit gauge would
cause the gauge to be improperly clipped if it was close enough to the
right or bottom edge of the texture.
git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10965 387891d4-d844-0410-90c0-e4c51a9137d3