In order to be able to support 16bit canvases on 32bit screens and 32bit
canvases on 16bit screens we need to handle format conversion when drawing
RedPixmaps.
The way this works now for X11 is that we only have one PIXELS_SOURCE_TYPE
for pixmaps, which always has a pixman_image_t for the data, but additionally
it has an XImage (shared mem or not) if the screen the pixmap was created
for (i.e. an explicit one or the default screen) has the same format as
the pixmap.
When we draw a pixmap on a drawable we have two variants. If the pixmap
has a XImage and it matches the format of the target drawable then we
just X(Shm)PutImage it to the drawable. If the formats differ, then we
create a temporary XImage and convert into that before drawing it to
the screen.
Right now this is a bit inefficient, because we always allocate a new
temporary image when converting. We want to add some caching here, but
at least this lets things work again.
We need to know the format for other drawables too (like for instance
the native format of a window), so we're pushing this down.
This changes a bunch of references to be RedDrawable::, but not all.
The the old RedPixmap:: references still work, but will be phased out.
We now support 16bit format pixmaps as well as the old ones. Including
both 555 and 565 modes.
We drop the palette argument for pixmap construction as it was only
used for black/white anyway.
Canvas creation is simplified so that there is no separate set_mode
state. Canvases are already created in the right mode and never change.
