feat: Add constants for RGTC compressed textures (three-types#322)

Author: 0b5vr

types/three/src/constants.d.ts

@@ -266,6 +266,12 @@ export const RGBA_ASTC_12x12_Format: CompressedPixelFormat;
 // BPTC compressed texture formats
 export const RGBA_BPTC_Format: CompressedPixelFormat;
 
+// RGTC compressed texture formats
+export const RED_RGTC1_Format: CompressedPixelFormat;
+export const SIGNED_RED_RGTC1_Format: CompressedPixelFormat;
+export const RED_GREEN_RGTC2_Format: CompressedPixelFormat;
+export const SIGNED_RED_GREEN_RGTC2_Format: CompressedPixelFormat;
+
 // Loop styles for AnimationAction
 export enum AnimationActionLoopStyles {}
 export const LoopOnce: AnimationActionLoopStyles;