Look for GL 1.1 functions in win32's OpenGL32.dll.

The windows ABI is GL 1.1, not 1.0.  You get NULL pointers for 1.1
functions when calling wglGetProcAddress().  Fixes #47.

src/dispatch_common.c

@@ -527,7 +527,7 @@ void *
 epoxy_get_core_proc_address(const char *name, int core_version)
 {
 #ifdef _WIN32
-    int core_symbol_support = 10;
+    int core_symbol_support = 11;
 #elif defined(ANDROID)
     /**
      * All symbols must be resolved through eglGetProcAddress