Fix AMD/Mesa support in ubuntu 16.04+

[cjritola]

Starting with ubuntu 16.04, fglrx is discontinued and programs must use Mesa.

Fortunately Mesa is 3.3-capable.

Unfortunately, it is failing to verify a sound-shader and gives no error message detailing what the issue is in the code. This fix will likely involve verification outside the Mesa driver to isolate the problem.

[cjritola]

Update: The sound shader is written against GL ES2 (GLSL 1.00). AMD/Mesa accepts the shader if rewritten to GLSL 1.10, however there are still internal errors thrown by the driver later and rendering will not proceed.