Since the sequential copies heuristic went (50696c5b1f), every partial readback
waits for the GPU. Motorstorm reads parts of the screen about 18 times a frame,
and ran about 75% slower than 1.20.4: 5403 blocking readbacks in 10 emulated
seconds, against 156.
With the flag, the first partial color readback of a buffer reads all of it into
a copy outside PSP memory, and the pieces until the next render to the buffer
are copied from there. Unlike the old heuristic, only what the game asked for is
written to PSP memory. Motorstorm is back to 153 blocking readbacks and 1.20.4's
speed, with identical output. Grand Knights History, which the heuristic was
added for, gets the flag too.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
