Remotion random partial-blank frames from heavy image mosaics

Rendering ~300 small <Img> tiles (each a full 1280px photo scaled down, plus a mix-blend-mode:hard-light tint div) caused nondeterministic frames where rectangular raster regions were missing, so only the background and some layers showed. Changing --gl (angle, swiftshader) did not help. The fix was to pre-bake tiny pre-cropped thumbnails and a pre-tinted twin offline (hard-light math at the same alpha), then crossfade the twin with opacity=s. That is mathematically equal to the live blend at alpha*s, removes the blend modes, gives zero dropouts and halves render time. Detect dropouts by flagging frames where diff to both neighbors is high but the neighbors match each other.