Define missing OpenGL extensions for Windows

While this is not the 100% correct way, it's much cleaner than just
importing th whole header files.

src/refresh/header/local.h

@@ -39,6 +39,20 @@
  #define GL_COLOR_INDEX8_EXT GL_COLOR_INDEX
 #endif
 
+#ifndef GL_EXT_texture_filter_anisotropic
+ #define GL_TEXTURE_MAX_ANISOTROPY_EXT 0x84FE
+ #define GL_MAX_TEXTURE_MAX_ANISOTROPY_EXT 0x84FF
+#endif
+
+#ifndef GL_ARB_multitexture
+ #define GL_TEXTURE0_ARB 0x84C0
+ #define GL_TEXTURE1_ARB 0x84C1
+#endif
+
+#ifndef GL_VERSION_1_3
+#define GL_TEXTURE1 0x84C1
+#endif
+
 #define TEXNUM_LIGHTMAPS    1024
 #define TEXNUM_SCRAPS       1152
 #define TEXNUM_IMAGES       1153

src/refresh/r_image.c

@@ -94,7 +94,9 @@ gltmode_t gl_solid_modes[] = {
 	{ "GL_RGB5", GL_RGB5 },
 	{ "GL_RGB4", GL_RGB4 },
 	{ "GL_R3_G3_B2", GL_R3_G3_B2 },
+#ifdef GL_RGB2_EXT
 	{ "GL_RGB2", GL_RGB2_EXT },
+#endif
 };
 
 #define NUM_GL_SOLID_MODES ( sizeof ( gl_solid_modes ) / sizeof ( gltmode_t ) )