gallium: do a proper implementation of GL_OES_read_format

Examine the currently bound color buffer's format to see if there's a good
format/type match.

src/mesa/state_tracker/st_context.c

@@ -49,6 +49,7 @@
 #include "st_cb_drawtex.h"
 #endif
 #include "st_cb_fbo.h"
+#include "st_cb_get.h"
 #if FEATURE_feedback
 #include "st_cb_feedback.h"
 #endif
@@ -293,6 +294,7 @@ void st_init_driver_functions(struct dd_function_table *functions)
    st_init_rasterpos_functions(functions);
 #endif
    st_init_fbo_functions(functions);
+   st_init_get_functions(functions);
 #if FEATURE_feedback
    st_init_feedback_functions(functions);
 #endif