func BeginFrame(width, height int, devicePixelRatio float32) {
w := C.GLsizei(float32(width) * devicePixelRatio)
h := C.GLsizei(float32(height) * devicePixelRatio)
C.glViewport(0, 0, w, h)
C.glClearColor(0, 0, 0, 1)
C.glClear(C.GL_COLOR_BUFFER_BIT | C.GL_DEPTH_BUFFER_BIT | C.GL_STENCIL_BUFFER_BIT)
C.nvgBeginFrame(vg, C.int(width), C.int(height), C.float(devicePixelRatio))
}
func (c *Context) BeginFrame(windowWidth, windowHeight int, devicePixelRatio float32) {
C.nvgBeginFrame(c.ctx,
(C.int)(windowWidth), (C.int)(windowHeight),
(C.float)(devicePixelRatio))
}