Make mixed samples contingent on auxiliary extensions

src/gpu/gl/GrGLInterface.cpp

Index: src/gpu/gl/GrGLInterface.cpp
diff --git a/src/gpu/gl/GrGLInterface.cpp b/src/gpu/gl/GrGLInterface.cpp
index 89f1f9ba65cda6671eff1ee548483b9ecb8c85ee..8bbd2f3b1b77c508a025269de66ac03988eaa6b7 100644
--- a/src/gpu/gl/GrGLInterface.cpp
+++ b/src/gpu/gl/GrGLInterface.cpp
@@ -528,6 +528,12 @@ bool GrGLInterface::validate() const {
         }
     }
 
+    if (fExtensions.has("GL_EXT_raster_multisample")) {
+        if (NULL == fFunctions.fRasterSamples) {
+            RETURN_FALSE_INTERFACE
+        }
+    }
+
     if (fExtensions.has("GL_NV_framebuffer_mixed_samples")) {
         if (NULL == fFunctions.fCoverageModulation) {
             RETURN_FALSE_INTERFACE