Fix oval rendering in vulkan

src/gpu/vk/GrVkCaps.cpp

Index: src/gpu/vk/GrVkCaps.cpp
diff --git a/src/gpu/vk/GrVkCaps.cpp b/src/gpu/vk/GrVkCaps.cpp
index a9ad05c8997970e7aaa3986faa2e2a95b5151363..d8ac8757e5c378315dd14b45f58a764634e47c2c 100644
--- a/src/gpu/vk/GrVkCaps.cpp
+++ b/src/gpu/vk/GrVkCaps.cpp
@@ -120,6 +120,8 @@ void GrVkCaps::initGLSLCaps(const GrVkInterface* interface, VkPhysicalDevice phy
             glslCaps->fConfigTextureSwizzle[i] = GrSwizzle::RGBA();
         }
     }
+
+    glslCaps->fShaderDerivativeSupport = true;

[egdaniel, 2016/02/25 21:07:23]

This should always be supported and I don't think we need to enable an extension
in the shader since SPIR-V has derivative support directly

 }
 
 static void format_supported_for_feature(const GrVkInterface* interface,