LibGfx: Give Bitmap a scale factor

Gfx::Bitmap can now store its scale factor. Normally it's 1, but
in high dpi mode it can be 2.

If a Bitmap with a scale factor of 2 is blitted to a Painter with
scale factor of 2, the pixels can be copied over without any resampling.
(When blitting a Bitmap with a scale factor of 1 to a Painter with scale
factor of 2, the Bitmap is painted at twice its width and height at
paint time. Blitting a Bitmap with a scale factor of 2 to a Painter with
scale factor 1 is not supported.)

A Bitmap with scale factor of 2 reports the same width() and height() as
one with scale factor 1. That's important because many places in the
codebase use a bitmap's width() and height() to layout Widgets, and all
widget coordinates are in logical coordinates as well, per
Documentation/HighDPI.md.

Bitmap grows physical_width() / physical_height() to access the actual
pixel size. Update a few callers that work with pixels to call this
instead.

Make Painter's constructor take its scale factor from the target bitmap
that's passed in, and update its various blit() methods to handle
blitting a 2x bitmap to a 2x painter. This allows removing some gnarly
code in Compositor. (In return, put some new gnarly code in
LibGfxScaleDemo to preserve behavior there.)

No intended behavior change.

Documentation/HighDPI.md

@@ -18,7 +18,7 @@ Integer scale factors are needed in any case so let's get that working first. Ac
 Desired end state
 -----------------
 
-- All rects  (Window and Widget rects, mouse cursor) are in "logical" coordinates, which is the same as pixels at 1x scale, as much as possible.
+- All rects  (Window and Widget rects, mouse cursor, even bitmap sizes) are in "logical" coordinates, which is the same as pixels at 1x scale, as much as possible.
 - If something needs to be in pixels, its name starts with `physical_`. Physical coordinates should as much as possible not cross API boundaries.
 - Jury's still out if logical coordinates should stay ints. Probably, but it means mouse cursor etc only have point resolution, not pixel resolution
 - We should have something that can store a collection of (lazily-loaded?) bitmaps and fonts that each represent a single image / font at different scale levels, and at paint time the right representation is picked for the current scale