LCLImager demo: images with all alpha=0 are displayed as black

[galfar]

Images with whole alpha channel at zero are displayed as completely black image (at least in Windows). Set just one pixel to different alpha and it's displayed correctly (blended over empty window). Could be an underlying issue with LCL picture support used for display.

This would look as "empty window" in LCLImager so that usual checkerboard as a background should also be implemented.

test-some-alpha:

test-cleared-alpha (shows white here as it should :):