OGL: Handle case where both constant alpha and logic op is enabled

[stenzek]

Fixes kirby-shadows on GL, albeit a far from perfect solution, but I can't think of any other way to do this. Will only impact performance (by the extra draw call) when logic op and dstalpha is enabled, which was incorrect before anyway.

The idea is basically abusing the alpha-only pass for the lack-of-dual-source-blend fallback to write the constant alpha value to the framebuffer when logic ops are enabled, since the logic op applies to all channels, and blending and logic ops are mutually exclusive.

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This change is

[degasus]

The comment is a bit recursive, as it explain the code for dual pass destAlpha with dual pass destAlpha ;)

but LGTM

[dolphin-emu-bot]

FifoCI detected that this change impacts graphical rendering. Here are the behavior differences detected by the system:

• kirby-shadows on ogl-lin-intel: diff
• kirby-shadows on ogl-lin-mesa: diff
• kirby-shadows on ogl-lin-nv: diff
• mario-baseball-shadows on ogl-lin-nv: failed to render
• simpsons-game on ogl-lin-nv: diff

_{^{automated-fifoci-reporter}}

[Fallcrest]

@stenzek Why did the Mario Baseball test fail?

[JMC47]

random failure