Move the SetLayeredWindowAttributes call to immediately after window
construction, and initialize it with per-pixel transparency settings, no
color key and no global blending. The render pipeline will perform alpha
blending during compositing.

Cleaned up the DWM acrylic API calls some, to explicitly set to the
three appropriate modes depending on opaque, transparent or blurred
settings. The API internally hides versioning concerns from the caller.

Set the window class background color to black, this prevents a
flashbang on slow startup, e.g. debug builds on a heavily loaded system.

The outcome is that the window no longer receives paint demands for
underlying window updates, while also having per-pixel transparency -
opaque theme elements are now correctly opaque. The transparency
settings are now portable across windows and macOS having mostly similar
outcomes (modulo palette differences). Small fonts may still appear to
be alpha blended - this seems to be in the glyph atlas, their pixels are
not actually opaque. Larger fonts (or higher DPIs) don't suffer this and
are as opaque as expected. Layering the window atop one that is
rendering at 120fps, the editor window can drop to its 8fps idle state,
while still being composited with 120fps alpha blend in the background,
in both blur and transparent modes.

Updates #20400

Release Notes:

- Improved transparency on Windows to be more efficient, support fully
opaque elements and more closely match other platforms.