The water shader's `u_screen`, and the reflection target's size, were both taken
from gl_w/gl_h - the drawable. But gl_FragCoord in that shader runs over the
scene target, which is post_w x post_h. They match only at a render scale of 1;
at anything less the refraction and depth reads landed in the wrong corner of
the frame and the lake showed a squashed copy of it instead of its own bed.
Both now come from the scene target. The reflection is sized from it too, which
also stops it paying for pixels the water never samples. R3D_DUMP_REFL reads the
target's own w/h rather than recomputing them.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>