Cache the prepared background bitmap to avoid re-rendering on every paint

[zencode1]

Summary

• StaticBackground/TileBackground previously called PrepareBackGroundPicture on every paint, which allocates a new TBitmap, resizes it, and re-renders the background graphic (including a MaskBlt/transparency pass) even though the result is identical from one paint to the next.
• This PR adds FBackgroundPrepared (bitmap + the BackgroundColor/Transparent settings it was rendered with) and a new GetBackgroundBitmap helper that only re-renders when the background color or transparency setting has changed since the last paint; otherwise it returns the cached bitmap. The cache is also invalidated whenever the background picture changes (Background.OnChange).
• Noticeably reduces CPU/GDI cost for scrolling and repeated repaints on trees with a background image, since the expensive preparation step now runs only on actual settings changes instead of every paint.

Notes for reviewers

• PrepareBackGroundPicture is unchanged - the existing TImage-style BackGroundImageTransparent contract (issue Background image should support transparency #662) is preserved.
• The cached bitmap is freed and recreated (not resized in place) whenever it needs to be regenerated, so there's no risk of stale pixels bleeding through on a cache miss.
• The final commit ("Clean up indentation") is whitespace-only and optional - feel free to skip or squash it if you'd rather keep history minimal.

Concerns

• Since Background is a public property, and OnChange is a single public callback, if a consumer assigns Background.OnChange after construction, it replaces this handler. Any subsequent changes then leave FBackgroundPrepared.Bitmap stale, and the tree continues painting the old image. To resolve this, we would need a more robust notification mechanism. That is beyond the scope of this PR.

Test plan

• Verified background image rendering (static and tiled modes) is visually unchanged before/after this change, including with BackGroundImageTransparent on/off and non-bitmap graphics (icon/PNG).
• Verified switching the background image, background color, or BackGroundImageTransparent at runtime still updates the displayed background correctly.
• Built VirtualTreesDevelopment.groupproj (Debug) - compiles clean, 0 warnings on the modified unit.

[zencode1]

After making the PR, I noticed that it is possible for aBkgColor to be a system color which could leave the cached image stale after a style change. To resolve this issue, I added a call to ColorToRGB.

Additionally, I added a "Concerns" section to my description regarding a possible problem if a consumer makes use of Background.OnChange.

[Daniel-Tr]

As for the concerns mentioned in the PR:

To resolve this, we would need a more robust notification mechanism. That is beyond the scope of this PR.

This is a potential breaking change (even though most likely not very likely), so I think we should address it first.