Add support for point sprites.

simgear/screen/extensions.hxx

@@ -568,6 +568,26 @@ typedef void (*glXQueryDrawableProc) (Display *, GLXDrawable, int, unsigned int
 #define GL_LUMINANCE_ALPHA_FLOAT16_ATI				0x881F
 #endif
 
+/*
+ * ARB point sprite
+ */
+#ifndef GL_ARB_point_sprite
+#define GL_ARB_point_sprite 1
+#define GL_POINT_SPRITE_ARB               0x8861
+#define GL_COORD_REPLACE_ARB              0x8862
+#endif
+
+#ifndef GL_NV_point_sprite
+#define GL_NV_point_sprite 1
+#define GL_POINT_SPRITE_NV                0x8861
+#define GL_COORD_REPLACE_NV               0x8862
+#define GL_POINT_SPRITE_R_MODE_NV         0x8863
+#endif
+
+#ifndef GL_VERSION_2_0
+#define GL_POINT_SPRITE                   0x8861
+#endif
+
 /*
  * ARB_vertex_program
  */
@@ -880,5 +900,6 @@ typedef void (APIENTRY * glProgramParameter4fvNVProc) (GLenum target, GLuint ind
 }
 #endif
 
+
 #endif // !__SG_EXTENSIONS_HXX