RFR: 8232210: Update Mesa 3-D Headers to version 19.2.1

[Kevin Rushforth]

This PR updates the header files we use the build the OpenGL ES2 pipeline to Mesa 19.2.1. See [this review thread](https://mail.openjdk.java.net/pipermail/2d-dev/2019-October/010372.html) for the equivalent change that is under review for Java2D.

The updates to the `gl.h` and `glx.h` files are large, since we are many, many years behind.

The `*ext.h` header files were updated fairly recently, so those diffs are not large.

Previously we used to get the `*ext.h` headers from Khronos, but now we get all the headers from the Mesa project.

This reduces the number of upstream sources we need to monitor.

I note that with this update, the `glxext.h` and `wglext.h` files are slightly older in the Mesa bundle than in Khronos, but the differences are not relevant to FX.

I did a full build and test on Mac and Linux and a sanity build (with `-PINCLUDE_ES2=true`) on Windows. I also verified that the build artifacts are unchanged.

As with the Java2D change, the licensing terms are the same as before, but since we no longer get files directly from Khronos, the `opengl_fx.md` file is gone and the `mesa3d.md` is updated as required to mention these files.

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Commits:
 - 7a520adc: 8232210: Update Mesa 3-D Headers to version 19.2.1

Changes: https://git.openjdk.java.net/jfx/pull/26/files
 Webrev: https://webrevs.openjdk.java.net/jfx/26/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8232210
  Stats: 1515 lines in 8 files changed: 1076 ins; 269 del; 170 mod
  Patch: https://git.openjdk.java.net/jfx/pull/26.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/26/head:pull/26

PR: https://git.openjdk.java.net/jfx/pull/26