Enable viewport tests on OpenGL ES.

Also implement a workaround for missing glViewportIndexedf().

BUG=dawn:580,dawn:597

Change-Id: I618161ae9750925c1c892123607db84563f0869c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/34721
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/opengl/CommandBufferGL.cpp

@@ -1173,7 +1173,15 @@ namespace dawn_native { namespace opengl {
 
                 case Command::SetViewport: {
                     SetViewportCmd* cmd = mCommands.NextCommand<SetViewportCmd>();
+                    if (gl.IsAtLeastGL(4, 1)) {
                         gl.ViewportIndexedf(0, cmd->x, cmd->y, cmd->width, cmd->height);
+                    } else {
+                        // Floating-point viewport coords are unsupported on OpenGL ES, but
+                        // truncation is ok because other APIs do not guarantee subpixel precision
+                        // either.
+                        gl.Viewport(static_cast<int>(cmd->x), static_cast<int>(cmd->y),
+                                    static_cast<int>(cmd->width), static_cast<int>(cmd->height));
+                    }
                     gl.DepthRangef(cmd->minDepth, cmd->maxDepth);
                     break;
                 }

src/tests/end2end/ViewportOrientationTests.cpp

@@ -68,4 +68,5 @@ DAWN_INSTANTIATE_TEST(ViewportOrientationTests,
                       D3D12Backend(),
                       MetalBackend(),
                       OpenGLBackend(),
+                      OpenGLESBackend(),
                       VulkanBackend());

src/tests/end2end/ViewportTests.cpp

@@ -213,4 +213,5 @@ DAWN_INSTANTIATE_TEST(ViewportTest,
                       D3D12Backend(),
                       MetalBackend(),
                       OpenGLBackend(),
+                      OpenGLESBackend(),
                       VulkanBackend());