clear method
Implementation
void clear(WebGLRenderer renderer,
[bool color = true, bool depth = true, bool stencil = true]) {
final currentRenderTarget = renderer.getRenderTarget();
for (int i = 0; i < 6; i++) {
renderer.setRenderTarget(this, i);
renderer.clear(color, depth, stencil);
}
renderer.setRenderTarget(currentRenderTarget);
}