Some weirdness encountered so far:

B_OP_INVERT seems to invert the dest alpha as well,
some other modes act weird on the dest alpha channel, I guess we should not copy that behaviour

B_OP_SELECT is originally behaving very strange. For BBitmp drawing, it doesn't seem to do anything, and if it does something for Stroke* and Fill* methods, it doesn't do what the BeBook says it would do. My implementation is more according to the BeBook, though for images, it is worth investigating.

B_OP_ALPHA (with B_ALPHA_OVERLAY) seems to *assign* the source alpha to the dest alpha, which makes absolutely no sense at all

The BView pen location appears to be integer coords, which spoils the pen location version of BView::DrawString() for rotated text.

Dest alpha is actually only interesting when drawing into a BBitmap. When drawing into the frame buffer, dest alpha is logically 255, it wouldn't even need to be assigned. Maybe we could save a few CPU cycles if we adjust the DrawingMode classes accordingly. For now, the modes work as expected when drawing into a BBitmap.


Current comparision timings:

drawing_mode:		B_OP_COPY
pen size:			 1.0
ellipse test:		 5.15
ellipse test AA:	13.90
pen size:			 2.0
ellipse test:		15.2
ellipse test AA:	11.0