Replace HALF_REFRESH with double FAST_REFRESH technique for the BW
pass when dithered letterbox fill is active. This avoids the e-ink
crosstalk and image corruption that occurred when HALF_REFRESH drove
large areas of dithered gray pixels simultaneously.
Revert the hash-based block dithering workaround (bayerCrossesBwBoundary,
hashBlockDither) back to standard Bayer dithering for all gray ranges,
since the root cause was HALF_REFRESH rather than the dithering pattern
itself.
Letterbox fill is now included in all three render passes (BW, LSB, MSB)
so the greyscale LUT treats letterbox pixels identically to cover pixels,
maintaining color-matched edges.
Co-authored-by: Cursor <cursoragent@cursor.com>
0fda9031fd
