The result is dst * (src + 2a), up to 3x the dst, which fixed function
blending can't reach: doubling alpha in the shader clamps it to 2x. MotorStorm
Arctic Edge and Tomb Raider Anniversary brighten their whole frame this way
and came out much too dark. Through mode draws are mostly full screen passes
that don't overlap themselves, so the framebuffer copy is safe there; 3D draws
keep the approximation (copies there caused overlap problems in Silent Hill).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
