If I want to set a particular pixel to a particular RGBA value, I can't unless I switch between drawing modes, which is inefficient.
If I use #PB_2ddrawing_AlphaChannel then the RGB doesn't get set.
If I use #PB_2ddrawing_default then the A doesn't get set.
If I use #PB_2ddrawing_alphablend it blends the RGBA value with the existing one (instead of just writing the four bytes), thus giving the wrong result (and also it must be much slower).
#PB_2DDrawing_AlphaClip obviously isn't suitable either.
I'd say this is a rather fundamental feature which should be added.