message: Culling and FrontFace: verify the framebuffer coordinate
  This patch is to verify the framebuffer coordinate in Dawn (and WebGPU).
  It shows that gl_FragCoord(0, 0) is at the top left.
  TEST=dawn_end2end_tests
diff: @@ -39,11 +39,14 @@ class CullingTest : public DawnTest {
  pipelineDescriptor.cVertexStage.module =
  utils::CreateShaderModule(device, dawn::ShaderStage::Vertex, vs);
  + // gl_FragCoord of pixel(x, y) in framebuffer coordinate is (x + 0.5, y + 0.5). And we use
  + // RGBA8 format for the back buffer. So (gl_FragCoord.xy - vec2(0.5)) / 255 in shader code
  + // will make the pixel's R and G channels exactly equal to the pixel's x and y coordinates.
  const char* fs =
  "#version 450\n"
  "layout(location = 0) out vec4 fragColor;"
  "void main() {\n"
  - " fragColor = vec4(1.0, 0.0, 0.0, 1.0);\n"
  + " fragColor = vec4((gl_FragCoord.xy - vec2(0.5)) / 255, 0.0, 1.0);\n"
  "}\n";
  pipelineDescriptor.cFragmentStage.module =
  utils::CreateShaderModule(device, dawn::ShaderStage::Fragment, fs);
  @@ -75,7 +78,7 @@ class CullingTest : public DawnTest {
  dawn::Texture colorTexture = Create2DTextureForTest(dawn::TextureFormat::RGBA8Unorm);
  utils::ComboRenderPassDescriptor renderPassDescriptor({colorTexture.CreateDefaultView()});
  - renderPassDescriptor.cColorAttachmentsInfoPtr[0]->clearColor = {0.0, 1.0, 0.0, 1.0};
  + renderPassDescriptor.cColorAttachmentsInfoPtr[0]->clearColor = {0.0, 0.0, 1.0, 1.0};
  renderPassDescriptor.cColorAttachmentsInfoPtr[0]->loadOp = dawn::LoadOp::Clear;
  dawn::CommandEncoder commandEncoder = device.CreateCommandEncoder();
  @@ -87,14 +90,16 @@ class CullingTest : public DawnTest {
  dawn::Queue queue = device.CreateQueue();
  queue.Submit(1, &commandBuffer);
  - constexpr RGBA8 kDrawingColor = RGBA8(255, 0, 0, 255);
  - constexpr RGBA8 kBackgroundColor = RGBA8(0, 255, 0, 255);
  + constexpr RGBA8 kTopLeftColor = RGBA8(0, 0, 0, 255);
  + constexpr RGBA8 kBottomRightColor = RGBA8(3, 3, 0, 255);
  + constexpr RGBA8 kBackgroundColor = RGBA8(0, 0, 255, 255);
  - RGBA8 kCCWTriangleColor = isCCWTriangleCulled ? kBackgroundColor : kDrawingColor;
  - EXPECT_PIXEL_RGBA8_EQ(kCCWTriangleColor, colorTexture, 0, 0);
  + RGBA8 kCCWTriangleTopLeftColor = isCCWTriangleCulled ? kBackgroundColor : kTopLeftColor;
  + EXPECT_PIXEL_RGBA8_EQ(kCCWTriangleTopLeftColor, colorTexture, 0, 0);
  - RGBA8 kCWTriangleColor = isCWTriangleCulled ? kBackgroundColor : kDrawingColor;
  - EXPECT_PIXEL_RGBA8_EQ(kCWTriangleColor, colorTexture, kSize - 1, kSize - 1);
  + RGBA8 kCWTriangleBottomRightColor =
  + isCWTriangleCulled ? kBackgroundColor : kBottomRightColor;
  + EXPECT_PIXEL_RGBA8_EQ(kCWTriangleBottomRightColor, colorTexture, kSize - 1, kSize - 1);
  }
  static constexpr uint32_t kSize = 4;
  