This fixes a crash if foreground color == background color.

Problem reported by Alexei.

* graph/gblender.h (GBLENDER_VARS): Split off initialization to...
(GBLENDER_VARS_SET): ...this new macro.
(GBLENDER_CHANNEL_VARS): Split off initialization to...
(GBLENDER_CHANNEL_VARS_SET): ...this new macro.

* graph/gblcolor.h, graph/gblhbgr.h, graph/gblhrgb.h,
graph/gblvbgr.h, graph/gblvrgb.h: Updated.