Use checker backdrop for footage with alpha channel in clip editor

Also use glaDrawPixelsAuto as a fallback method. Hopefully it'll
make using 2D textures for frames higher resolution than
GL_MAX_TEXTURE_SIZE.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55404 954f8c5b-7b00-dc11-b283-0030488c597c