shell: texture the launch splash from a pre-rendered sheet

[deblasis]

Replaces the run-time engraver with one transformed blit. The sheet is drawn
offline and shipped as an asset; the splash picks a crop, a zoom and an angle,
which is about a thousand GDI+ calls fewer on the path whose whole job is to be
on screen before the app is.

Varying the crop rather than the drawing is the better trade anyway. At this
contrast a different piece of the same sheet is indistinguishable from a
differently composed one, and it costs nothing.

Replaceable

A PNG at %APPDATA%\wintty\splash-texture.png is used instead of the shipped
one, in the directory the config and themes already live in. Presence is the
whole signal, so nothing has to work out whether the shipped sheet was
tampered with, and the two being in different places means an upgrade replaces
what it installed and never sees the user's copy.

An ordinary picture has no alpha channel and would come out as one flat slab of
ink, since the shipped sheet is a mask and alpha is what says how far towards
the ink a pixel goes. For those, brightness says it instead. Decided from the
pixel format rather than by reading pixels, because this runs before the app is
on screen.

Tested

LaunchTexture and LaunchTextureSource are pure, so the things that fail
invisibly get tested. A crop that runs off the sheet draws a hard edge across
the splash. A turned image smaller than its window leaves a bare corner at some
angles and window shapes but not others. A resolution order that came out
backwards would ignore a customisation silently, which looks like the feature
simply not working.

Two of the tests exist because the splash looked static in use and neither the
code nor the other tests explained why:

The seed is a clock, and a seeded Random turns a steadily moving seed into a
steadily moving first draw -- its first value is very nearly linear in its seed.
The angle came out as a slow sawtooth, about four degrees between launches a
couple of seconds apart, which reads as no rotation at all. The seed is now
avalanched before anything is drawn from it.

The turn range is wide enough to look like a different angle each time, but
strictly inside a half turn either way so the content is never inverted, and
short of vertical at both ends so it never stops reading as what it is.

Alpha

The sheet is a white-on-nothing mask tinted at draw time. The colour matrix maps
it straight onto the background and the ink rather than blending over the
background, and pins alpha to 1: UpdateLayeredWindow is called with an opaque
source, so nothing here may depend on GDI+ blending having preserved it. Checked
offscreen on light and dark backgrounds, minimum alpha 255.