mesa: Include mtypes.h in files that use gl_context struct.

Directly include mtypes.h if a file uses a gl_context struct. This
allows future removal of headers that are not strictly necessary but
indirectly include mtypes.h for a file.

src/mesa/main/texenv.c

@@ -34,6 +34,7 @@
 #include "main/context.h"
 #include "main/enums.h"
 #include "main/macros.h"
+#include "main/mtypes.h"
 #include "main/texenv.h"
 #include "main/texstate.h"