glx: Add an optional function call for getting the DRI driver interface.

The previous interface relied on a static struct, which meant that the
driver didn't get a chance to edit the struct before the struct got used.
For megadrivers, I want struct specific to the driver being loaded.

v2: Fix the prototype in the docs (caught by Marek).  Since the driver
    name was in the function, we didn't need to also pass it in.
v3: Fix asprintf error checking (caught by Matt's gcc).

Reviewed-by: Matt Turner <mattst88@gmail.com> (v1)
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>

include/GL/internal/dri_interface.h

@@ -487,6 +487,19 @@ struct __DRIuseInvalidateExtensionRec {
  */
 #define __DRI_DRIVER_EXTENSIONS "__driDriverExtensions"
 
+/**
+ * This symbol replaces the __DRI_DRIVER_EXTENSIONS symbol, and will be
+ * suffixed by "_drivername", allowing multiple drivers to be built into one
+ * library, and also giving the driver the chance to return a variable driver
+ * extensions struct depending on the driver name being loaded or any other
+ * system state.
+ *
+ * The function prototype is:
+ *
+ * const __DRIextension **__driDriverGetExtensions_drivername(void);
+ */
+#define __DRI_DRIVER_GET_EXTENSIONS "__driDriverGetExtensions"
+
 /**
  * Tokens for __DRIconfig attribs.  A number of attributes defined by
  * GLX or EGL standards are not in the table, as they must be provided