[Cin] Improved Gradient Plugin

[Phyllis Smith]

Today's checkin was to get the Gradient Plugin (as often used in masking)
to create the same results when using the X11 driver and the X11-OpenGL
video driver.

A test version for Ubuntu16 is at:

https://www.cinelerra-gg.org/download/testing/cinelerra-5.1-ub16-x86_64-static.txz
which contains all of the bug fixes added since May 31.   gg/Phyllis
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