[OpenJDK 2D-Dev] RFR: 8232154: Update Mesa 3-D Headers to version 19.2.1

[Phil Race]

Bug: https://bugs.openjdk.java.net/browse/JDK-8232154
Webrev: http://cr.openjdk.java.net/~prr/8232154/

This is updating the header files we use to build the OpenGL code on all 
platforms.

It is quite a large update since we were many, many years behind.

Previously we used to get the "ext" 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.

There is one new header file that the "ext" headers now depend upon.
All includes of another file reference the J2D_GL location where we store
all these files.

Search for "JDK modification" comments to find these updates

There is a JDK modification in glxext.h to get it to build on Solaris
Details in the bug report and reported upstream.

Now builds fine and runs without issue in limited (SwingSet + J2Ddemo) 
testing.

The licensing is really all the same but since we no longer get files
directly from Khronos the opengl.md file is gone and the mesa3d.md
is updated as required by the lawyers to mention these files.

-phil.