gltrace: Intercept libGLX.so.

There's probably more to support glvnd than intercepting libGLX.so, as
applications can call libOpenGL.so too.  But this is enough for
applications which obtain OpenGL symbols through libGLX.so's
glXGetProcAddress(ARB).

Fixes #829

docs/USAGE.markdown

@@ -134,6 +134,10 @@ ordinary `libGL.so` and injecting it via `LD_LIBRARY_PATH`:
     export TRACE_LIBGL=/path/to/real/libGL.so.1
     /path/to/application
 
+_NOTE: This `LD_LIBRARY_PATH` approach will not work with applications which
+call [glvnd](https://gitlab.freedesktop.org/glvnd/libglvnd#architecture)
+components directly._
+
 If you are an application developer, you can avoid this either by linking with
 `-Bsymbolic` flag, or by using some unique prefix for your function pointers.
 

wrappers/dlsym.cpp

@@ -95,6 +95,7 @@ extern void * _libGlHandle;
 enum LibClass {
     LIB_UNKNOWN = 0,
     LIB_GL,
+    LIB_GLX,
     LIB_EGL,
     LIB_GLES1,
     LIB_GLES2,
@@ -114,6 +115,11 @@ classifyLibrary(const char *pathname)
         return LIB_GL;
     }
 
+    if (strcmp(filename, "libGLX.so") == 0 ||
+        strcmp(filename, "libGLX.so.0") == 0) {
+        return LIB_GLX;
+    }
+
 #ifdef EGLTRACE
     if (strcmp(filename, "libEGL.so") == 0 ||
         strcmp(filename, "libEGL.so.1") == 0) {