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alpha: doesn't correctly work with black or white in custom mode

Submitted by Andreas Frisch @fraxinas

Link to original bug (#766060)

Description

reference command - replace yellow bar in test pattern 0 with circles from other videotestsrc, works as expected:
gst-launch-1.0 videotestsrc ! alpha method=3 target-r=255 target-g=255 target-b=0 ! videomixer sink_0::zorder=1 sink_1::zorder=0 name=mixer ! videoconvert ! gtksink videotestsrc pattern=11 ! mixer.

test-commands, white stripe is supposed to be transparent, in fact all others are :)
gst-launch-1.0 videotestsrc ! alpha method=3 target-r=255 target-g=255 target-b=255 ! videomixer sink_0::zorder=1 sink_1::zorder=0 name=mixer ! videoconvert ! gtksink videotestsrc pattern=11 ! mixer.

same exact symptom with black:
gst-launch-1.0 videotestsrc ! alpha method=3 target-r=0 target-g=0 target-b=0 ! videomixer sink_0::zorder=1 sink_1::zorder=0 name=mixer ! videoconvert ! gtksink videotestsrc pattern=11 ! mixer.

slomo suggested it may have something to do with alpha's internal HSV conversion. this sounds likely since pure black and white don't have a defined hue value. i'll take a look

Version: 1.8.1