Hi,

fringe bitmaps currently don't respect the frame parameter
'alpha-background', i.e., their background is opaque. But since the
fringe itself *does* respect 'alpha-background', fringe bitmaps should
do the same. This patch fixes that. See attached screenshots for the
effect.
The alpha-background used here was set with:
  (set-frame-parameter nil 'alpha-background 50)
  
I am not sure which other platforms support 'alpha-background', and
unfortunately I am not able to test with those. This patch applies to
the pgtk and X Linux builds.

The change is altering the parameter 'respect_alpha_background' in
{pgtk,x}_cr_draw_image - while this name sounds quite generic, I didn't
find any other usages of this function than drawing fringe bitmaps. I
hope I didn't miss any.

I hope that I'm following the conventions here correctly - please let me
know if I can improve anything!

Best
Nate