如何在通常的iOS应用(基于UIWindow)中使用GPU加速的SkCanvas？

EAGLContext *context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
[EAGLContext setCurrentContext:context];

GrContext* grContext = SkGpuDevice::CreateGLContext(context);
GrBackendRenderTargetDesc desc;
desc.fWidth = width;
desc.fHeight = height;
desc.fConfig = kSkia8888_GrPixelConfig;
desc.fOrigin = kBottomLeft_GrSurfaceOrigin;
desc.fSampleCnt = 0;
desc.fStencilBits = 8;
desc.fRenderTargetHandle = 0;
sk_sp<SkSurface> surface(SkSurface::MakeFromBackendRenderTarget(grContext, desc, nullptr));

sk_sp<SkSurface> surface(SkSurface::MakeFromBackendRenderTarget(grContext, desc, nullptr));
sk_sp<SkCanvas> canvas(surface->getCanvas());

canvas->clear(SK_ColorWHITE);
SkPaint paint;
paint.setColor(SK_ColorBLUE);
canvas->drawRect(SkRect::MakeXYWH(0, 0, 100, 100), paint);

CALayer* layer = [[CALayer alloc] init];
layer.bounds = CGRectMake(0, 0, width, height);
layer.position = CGPointMake(width/2, height/2);
layer.contents = (__bridge id) surface->getBackendTexture(false);
[window.layer addSublayer:layer];