correct backing scale on Retina canvas & derived contexts

One consequence is that drawing with a pen of width 0 into
a canvas% object on a Retina display draws a line that is
half as wide as a pen of width 1.
dc<%>: add alignment scale

Partly reverts d8a438231f by making the alignment scale always default
to 1, but allows the "fix" (for programs that was drawing to a bitmap
with backing scale 2 to be like drawing with scale 2 into a bitmap
with backing scale 1) by setting the alignment scale to 2 for a
drawing destination that has a backing scale of 2.
add history notes to `racket/draw` docs

Add notes that should have been included with
90a534f304 and 5e903441a4.
add history notes to `images` docs

Add notes that should have been included with 4ee266fd97.
images/icons: add `close-icon`

Also add a `#:thickness` option to `x-icon`.
mrlib/close-icon: revise, especially for @2x

Modernize by using alpha instead of constructing masks, etc.
image-snip%: preserve bitmap backing scale on save & load