clear method
Implementation
void clear([bool color = true, bool depth = true, bool stencil = true]) {
int bits = 0;
if (color) bits |= _gl.COLOR_BUFFER_BIT;
if (depth) bits |= _gl.DEPTH_BUFFER_BIT;
if (stencil) bits |= _gl.STENCIL_BUFFER_BIT;
_gl.clear(bits);
}