Fix regression in viewport background image transparency

Was caused by recent changes to automatically switch between
glaDrawPixelsTex and glaDrawPixelsSafe depending on resolution.

glaDrawPixelsSafe could not be sued for viewport because it'll
miss alpha transparency.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55415 954f8c5b-7b00-dc11-b283-0030488c597c