RFR: 8311492: FontSmoothingType LCD produces wrong color when transparency is used [v3]

[Kevin Rushforth]

> JavaFX LCD text rendering (aka sub-pixel antialiasing) uses a pixel shader and alpha blending. The alpha channel is used is ways that interfere with its use for transparency. The existing logic checks that the current blend equation is SRC_OVER and that the surface is opaque, and that we are rendering using a Paint of type Color. It fails to check that the text color is opaque. When it isn't, the resulting alpha value is not preserved, even in the middle of the filled portion of the text, resulting in a visually noticeable difference in color.
> 
> ![transparent-lcd-text](https://github.com/openjdk/jfx/assets/34689748/81f9503c-c706-44d8-b4db-6b47f0eb5fea)
> 
> The solution is to add the missing check for alpha == 1 to the test that checks whether we can use LCD text rendering. I note that Java2D falls back to gray scale when the text color is transparent for a similar reason.
> 
> I added a robot test that checks the color in the middle of the filled portion of a rendered text character and also checks that we use LCD for opaque colors and GRAY scale for transparent colors.

Kevin Rushforth has updated the pull request incrementally with one additional commit since the last revision:

  more review feedback

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1361/files
  - new: https://git.openjdk.org/jfx/pull/1361/files/62c768cf..ee1a621c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1361&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1361&range=01-02

  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1361.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1361/head:pull/1361

PR: https://git.openjdk.org/jfx/pull/1361