Define macros needed by 26bbfea.

The code used build fine on my workstation, since Mesa3D has the
required macros since ever. But on Windows gl.h is still limited
to OpenGL 1 (really?).

src/client/refresh/header/local.h

@@ -58,6 +58,14 @@
  #define GL_TEXTURE1 0x84C1
 #endif
 
+#ifndef GL_MULTISAMPLE
+#define GL_MULTISAMPLE 0x809D
+#endif
+
+#ifndef GL_MULTISAMPLE_FILTER_HINT_NV
+#define GL_MULTISAMPLE_FILTER_HINT_NV 0x8534
+#endif
+
 #define TEXNUM_LIGHTMAPS 1024
 #define TEXNUM_SCRAPS 1152
 #define TEXNUM_IMAGES 1153