Omnibus mesa 13.0.3

[jfirebaugh]

This build of mesa includes all the variants we might want to use in a single package:

• libGL (GLX, DRI, and Gallium LLVM based implementation)
• libEGL + libGLEs (EGL, X11, DRI, and Gallium LLVM based implementation)
• libOSMesa (Gallium LLVM based implementation)

This package is set up so that using it does not automatically trigger any particular library to be linked. The package user is responsible for supplying the appropriate -l arguments depending on the desired library:

• For GLX: -lGL -lX11
• For EGL: -lGLESv2 -lEGL -lgbm
• For OSMesa: -lOSMesa

LLVM is statically linked (fixes #280).

Other differences from previous packagings of mesa:

• It's built with CFLAGS=-g CXXFLAGS=-g, for debuggability.
• It's built with --enable-valgrind to try to cut down on valgrind false positives.
• It's built with --enable-glx (same as --enable-glx=dri), rather than --enable-glx=gallium-xlib, because the latter is incompatible with --enable-dri, which we need for EGL.
• Building with it won't add the mason mesa lib directory to the built output's rpath. You'll need to set LD_LIBRARY_PATH if you want to actually use the mason mesa .so's at runtime. (You probably want to do this in CI but use drivers with hardware acceleration in production.)

Important learnings obtained over the course of several days of investigation:

• Travis does not support the "drm" platform for EGL -- the "x11" platform must be used. I haven't yet tried "surfaceless" (Provide a surfaceless EGL headless backend mapbox-gl-native#7020).
• OSMesa works only with --enable-gallium-osmesa. The --enable-osmesa option produces rendering artifacts in GL Native. Basically, we want to build such that we use the Gallium llvmpipe software renderer no matter what library interface we use (GLX, EGL, or OSMesa).
• There's a bug in Travis's trusty node image that prevents GLX from working out of the box: GLX extension not available in node trusty image travis-ci/travis-ci#7198

cc @springmeyer @brunoabinader