OGL: Fall back to the old dual-source blending behaviour.

[CrossVR]

This allows us to still use dual-source blending in cases where it is required for accurate emulation and not just for small precision enhancements where it could crash some driver.

I also added OS X Intel to the bug, as those drivers seem to be affected by a similar bug. The OS X Nvidia drivers are not affected. I've tested it on my own MacBook Pro and confirmed this PR works correctly.

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This change is

[ligfx]

I appreciate the enthusiasm to get this fixed :) I'd like to talk about this some more. I've heard a lot about how much better dual-source blending is, will turning it off on OSX lead to a performance hit?

If so, I'm still interested in a solution like #4397, which would let us keep dual-source turned on. We could probably come up with some numbers to better quantify "compiling a bunch of extra shaders:" in the FIFO log I've been testing, for example, only 1/28 shaders is used both with dual-source blending and normal blending, and on a short run of Super Mario Sunshine I'm seeing 11/192 shaders get used with both options (tracking from ProgramShaderCache::SetShader, which seems like the right place to do so).

[CrossVR]

@ligfx This doesn't turn off dual-source blending, it just doesn't use it for minor blending precision anymore on drivers that can't handle it correctly. Dual-source blending is still used in cases where the feature is required for proper emulation.

[ligfx]

@Armada651 So no performance hit?

[CrossVR]

@ligfx No performance hit, just some minor color differences invisible to the naked eye.

[ligfx]

Okay great! Thanks for answering my questions.