Blender Anim player fixes:

- RGBA images (without alpha) now don't draw with alpha blending enabled
- Checkerboard backdrop for alpha images now shows on size of image
- Window always clears now, in very dark grey (so you can see black)
- On resizing window, the aspect of image is being kept, keeping the
  image always visible within the window frame
- Image now centers OK. 
- On pause, playback now halts (it was drawing same image all the time)




git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55138 954f8c5b-7b00-dc11-b283-0030488c597c