Comparer: make drag and pan interactions smoother

[chammoru]

Migrated from the legacy TODO list in Comparer/README.md.

Goal:

• Improve movement smoothness while dragging/panning in Comparer.

The old note was partially mojibake, but it appears to describe jerky movement during image dragging.

[chammoru]

Progress update

Two low-hanging-fruit optimisations have been applied (commit 227803f):

Done

• ComparerView::OnDraw — CDC/CBitmap for the off-screen buffer were allocated and freed on every paint call. They are now member variables (mMemDC / mMemBitmap) recreated only in OnSize when the canvas size changes.
• DrawDiffOverlay — Gdiplus::Pen and SolidBrush were constructed on every paint. They are now lazily created once per view (mDiffCellPen / mDiffDotBrush). SmoothingModeAntiAlias was also removed (expensive, and unnecessary for axis-aligned rectangles).

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Remaining work (higher impact, more involved)

1. Cache the diff-overlay bitmap; skip redraw during drag (medium)

DrawDiffOverlay walks every display cell with a nested loop + memcmp on every OnDraw call, even during a pan where the image content has not changed.

Proposed approach

• Keep a cached CBitmap mDiffOverlayBitmap the same size as the canvas.
• Re-render the overlay into it only when the frame or zoom level changes (not during pan).
• During OnDraw, just BitBlt the cached overlay on top of the main image.
• While mIsClicked (active drag), skip DrawDiffOverlay entirely; redraw it once on OnLButtonUp.

2. Skip ScaleRgbBuf during pan; BitBlt from a cached scaled bitmap (hard)

ScaleNearestNeighbor (or Interpolate) re-scales the source image on every OnDraw call, even though the zoom level (mN) is unchanged during a pan — only the viewport offset changes.

Proposed approach

• Keep a pre-scaled CBitmap mScaledImgBitmap at the current zoom level, sized to the full scaled image dimensions (mWDst × mHDst).
• Invalidate it when the frame content or mN changes; regenerate it off the UI thread (worker thread or on next idle).
• During OnDraw, replace the ScaleRgbBuf + SetDIBitsToDevice path with a single BitBlt from the cached bitmap at the current (mXDst, mYDst) offset.
• Memory trade-off: the cached bitmap can be large when zoomed in; cap it at the canvas size to bound memory usage.