racket/gui: DPI-aware on Windows

The `racket/draw` library is now independent of the screen resolution
on Windows. Font sizes in "points" are the only place where the
resolution mattered before, and now `racket/draw` assumes a
traditional 96dpi on Windows and Linux (and a traditional 72dpi
on Mac OS X).

Setting the scale for "text and other items" in Windows now adjusts
the backing scale of screen and canvas-compatible bitmaps, as well as
setting a scale on canvas drawing. Window and screen positions and
sizes are similarly scaled; for example, if the screen is 2048x1436
with text scaled by 200%, then `racket/gui` reports the display size
as 1024x768 (and the display backing scale as 2.0).

Backing scales of 1.25 and 1.5 are common for Windows. Rounding
associated with those scales could cause trouble for virtual -> actual
-> virtual conversions.
racket/gui win32: fix measurement of `message%` content

Strip away "&" before measuring.
racket/draw: compensate for roundoff in Windows GUI scaling

Roundoff can place a child of a container so that it's position plus
width extend beyond the container's width. For some controls, that
looks especially bad. Approximate precise scaling by having those
control sizes round down, instead of up, while leaving other kinds
of controls alone.
racket/draw: fix font metrics versus transformations

For a font with 'aligned hinting, the font map caches metric
information that depends on the destination transformation,
at least on Windows. Make the font-map cache sensitive to the
destination's current transformation.

This bug was exposed by support for DPI-aware GUIs on Windows,
but the problem was more general.
racket/gui win32: try to make pan gestures work

A pan gesture is turned into a WM_{H.V}SCROLL event in a
way that `racket/gui` did not recognize as a change to the
scrollbar. I'm not sure that this change fixes the problem,
but it seems worth a try.
racket/draw: remove Pango bug-repair hack for Windows

Bug reported as 649293 is apparently fixed, so the hack is
no longer needed.
racket/draw: constrain font-map repair to Windows

Adjust changes in 6bfad43429 to avoid creating more font maps than
necessary, especially on platforms other than Windows.
racket/gui Unix: avoid startup output