activeTexture( webglSlot ) {

		const { gl, currentTextureSlot, maxTextures } = this;

		if ( webglSlot === undefined ) webglSlot = gl.TEXTURE0 + maxTextures - 1;

		if ( currentTextureSlot !== webglSlot ) {

			gl.activeTexture( webglSlot );
			this.currentTextureSlot = webglSlot;

		}

	}
Makes the given texture unit active.

This method caches the state so `gl.activeTexture()` is only
called when necessary.

@param {number} webglSlot - The texture unit to make active.