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fix part of [2017702] OpenGL bugs with alpha values of 1.0 in the source during blending into 8888 buffers

when ONE / ONE_MINUS_SRC_ALPHA blending mode was used, the code wasn't saturating the color component.
the reason was that this mode is used for premltiplied alpha blending, however, if used with a non
premultiplied source, the color component would wrap.

unfortunately, this costs 6 extra cycles per pixels, however... "correctness" prevails.

this should not impact the UI since it's using h/w acceleration most of the time it also doesn't
impact games which should be using h/w GL. This change will slow the emulator down a bit.