Take the source alpha into account when doing a B_OP_INVERT so that drawing

bitmaps in B_OP_INVERT mode does not affect pixels where the source bitmap
was transparent, as noted in the BeBook. Not really sure I'm doing that right
though and probably needs looking into for B_OP_ERASE and B_OP_SELECT too.
Fixes bug #2421 though.

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@26067 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/servers/app/drawing/Painter/drawing_modes/DrawingModeInvert.h

@@ -133,7 +133,7 @@ blend_color_hspan_invert(int x, int y, unsigned len,
 	if (covers) {
 		// non-solid opacity
 		do {
-			if (*covers) {
+			if (*covers && colors->a > 0) {
 				if (*covers == 255) {
 					ASSIGN_INVERT(p);
 				} else {
@@ -142,22 +142,23 @@ blend_color_hspan_invert(int x, int y, unsigned len,
 			}
 			covers++;
 			p += 4;
-//					++colors;
+			++colors;
 		} while(--len);
 	} else {
 		// solid full opcacity
 		if (cover == 255) {
 			do {
-				ASSIGN_INVERT(p);
+				if (colors->a > 0) {
+					ASSIGN_INVERT(p);
+				}
 				p += 4;
-//						++colors;
+				++colors;
 			} while(--len);
 		// solid partial opacity
 		} else if (cover) {
 			do {
 				BLEND_INVERT(p, cover);
 				p += 4;
-//						++colors;
 			} while(--len);
 		}
 	}