Skip validating draw buffers when RASTERIZER_DISCARD is enabled.

src/libANGLE/validationES.cpp

@@ -2778,7 +2778,8 @@ const char *ValidateDrawStates(Context *context)
                 return kVertexShaderTypeMismatch;
             }
 
-            if (!context->getState().getBlendState().allChannelsMasked())
+            if (!context->getState().getRasterizerState().rasterizerDiscard &&
+                !context->getState().getBlendState().allChannelsMasked())
             {
                 // Detect that if there's active color buffer without fragment shader output
                 if (!ValidateFragmentShaderColorBufferMaskMatch(context))

src/tests/gl_tests/DrawBuffersTest.cpp

@@ -444,6 +444,11 @@ TEST_P(DrawBuffersWebGL2Test, TwoProgramsWithDifferentOutputsAndClear)
     setDrawBuffers(kMaxBuffers, allBufs);
     drawQuad(program, positionAttrib(), 0.5, 1.0f, true);
     ASSERT_GL_ERROR(GL_INVALID_OPERATION);
+    // Exception: when RASTERIZER_DISCARD is enabled.
+    glEnable(GL_RASTERIZER_DISCARD);
+    drawQuad(program, positionAttrib(), 0.5, 1.0f, true);
+    ASSERT_GL_NO_ERROR();
+    glDisable(GL_RASTERIZER_DISCARD);
     // Exception: when all 4 channels of color mask are set to false.
     glColorMask(false, false, false, false);
     drawQuad(program, positionAttrib(), 0.5, 1.0f, true);