getSupportedCompressedTextureFormats() {
		if (typeof supportedCompressedTextureFormats === "undefined") {
			const gl = this.gl;
			supportedCompressedTextureFormats = {
				astc:
					gl.getExtension("WEBGL_compressed_texture_astc") ||
					this._gl.getExtension("WEBKIT_WEBGL_compressed_texture_astc"),
				bptc:
					gl.getExtension("EXT_texture_compression_bptc") ||
					this._gl.getExtension("WEBKIT_EXT_texture_compression_bptc"),
				s3tc:
					gl.getExtension("WEBGL_compressed_texture_s3tc") ||
					this._gl.getExtension("WEBKIT_WEBGL_compressed_texture_s3tc"),
				s3tc_srgb:
					gl.getExtension("WEBGL_compressed_texture_s3tc_srgb") ||
					this._gl.getExtension("WEBKIT_WEBGL_compressed_texture_s3tc_srgb"),
				pvrtc:
					gl.getExtension("WEBGL_compressed_texture_pvrtc") ||
					this._gl.getExtension("WEBKIT_WEBGL_compressed_texture_pvrtc"),
				etc1:
					gl.getExtension("WEBGL_compressed_texture_etc1") ||
					this._gl.getExtension("WEBKIT_WEBGL_compressed_texture_etc1"),
				etc2:
					gl.getExtension("WEBGL_compressed_texture_etc") ||
					gl.getExtension("WEBKIT_WEBGL_compressed_texture_etc") ||
					gl.getExtension("WEBGL_compressed_texture_es3_0"),
			};
		}
		return supportedCompressedTextureFormats;
	}
return the list of supported compressed texture formats
@return {Object}