useShader(shader) {
		if (this.renderer.currentProgram !== shader.program) {
			this.flush();
			shader.bind();
			shader.setUniform("uProjectionMatrix", this.renderer.projectionMatrix);
			shader.setVertexAttributes(this.gl, this.attributes, this.vertexByteSize);

			this.currentShader = shader;
			this.renderer.currentProgram = this.currentShader.program;
		}
	}
Select the shader to use for compositing
@see GLShader
@param {GLShader} shader - a reference to a GLShader instance