Only gate really expensive block transfers behind the setting.

[hrydgard]

It's a long term goal of mine to mostly kill off the "block transfer" and "slow effects" settings.

As an early step, I'm proposing to redefine it to only affect the very slowest block transfers (readbacks).

I don't know what will be affected by this, but for most users who already have the setting enabled, there should be no difference. Possibly a strange game or too might slow down very slightly for users who like to play with it off, but should render more accurately and won't slow down all that much.

[hrydgard]

Like #11777, this makes a little bit of progress on #11610 .

The idea of doing this step by step is to get reports about bad performance regressions so we can fix them.

[unknownbrackets]

A user reported a performance hit:

• Device: GPD XD (Rockchip 3288 / 1.8 Ghz 4 core / Mali-T764 600mhz)
• OS: Android 4.4.4
• Backend: OpenGL
• Game: Brave Story: New Traveler
• Resolution: 2x, auto texture filtering
• Previously had full speed with block transfer enabled, but slow effects off (maybe it's not actually this PR despite range given...)

This particular game does annoying CLUT effects, blending / logic ops, flat shading, and render-to-self texture. I only remember block transfer being involved in the birds minigame...

-[Unknown]