PixelShaderGen: only use dual-source if bpmem requires dual-src blending

[ligfx]

This fixes Issue 9373: Depth issues on Mario Kart Double Dash.

There seems to be some interaction between having ocol1 in the fragment shader and calling glBindFragDataLocationIndexed: removing either one fixes the issue. Adding stronger dual source requirements to the fragment shader seemed like the cleaner option.

Before:

After:

[iwubcode]

Seems fine to me but @stenzek or @degasus should give it a second look.

[CrossVR]

This seems redundant, why not just add MacOS to the BUG_BROKEN_DUAL_SOURCE_BLENDING in the driver details and update the description to make it more general?

[degasus]

always doing dual source blend was on purpose, as we want to use the color as blend factors with 24 bit accuracy and write the color with rgb565 accuracy.

[kemenaran]

Just tested this PR on macOS 10.12: I can confirm this fixes issue Issue 9373 (Depth issues on Mario Kart Double Dash), and similar depth issues on Mario Kart Wii.

@ligfx thank you so much for your work on this!

[ghost]

This can help my issue (dark blue filter overlay)?

Android 11 Vulkan Mali-G52

[JosJuice]

@Gamemulatorer No.