Add support for GL_EXT_raster_multisample

src/gpu/gl/GrGLCaps.cpp

Index: src/gpu/gl/GrGLCaps.cpp
diff --git a/src/gpu/gl/GrGLCaps.cpp b/src/gpu/gl/GrGLCaps.cpp
index d95e10ba64d80d3c53ad73e48e725db38d299161..599e375c231b3e1f4e2992b163d20d2058ce98d5 100644
--- a/src/gpu/gl/GrGLCaps.cpp
+++ b/src/gpu/gl/GrGLCaps.cpp
@@ -306,11 +306,19 @@ void GrGLCaps::init(const GrContextOptions& contextOptions,
         fShaderCaps->pathRenderingSupport()) {
         fUsesMixedSamples = ctxInfo.hasExtension("GL_NV_framebuffer_mixed_samples") ||
                 ctxInfo.hasExtension("GL_CHROMIUM_framebuffer_mixed_samples");
-        // Workaround NVIDIA bug related to glInvalidateFramebuffer and mixed samples.
-        if (fUsesMixedSamples && (kNVIDIA_GrGLDriver == ctxInfo.driver() ||
-                                  kChromium_GrGLDriver == ctxInfo.driver())) {
-            fDiscardRenderTargetSupport = false;
-            fInvalidateFBType = kNone_InvalidateFBType;
+        if (fUsesMixedSamples) {
+            if (kNVIDIA_GrGLDriver == ctxInfo.driver() ||
+                kChromium_GrGLDriver == ctxInfo.driver()) {
+                // Workaround NVIDIA bug related to glInvalidateFramebuffer and mixed samples.
+                fDiscardRenderTargetSupport = false;
+                fInvalidateFBType = kNone_InvalidateFBType;
+            }
+            // Raster multisample is only useful to us when coverage modulation is also supported,
+            // as defined by mixed samples.
+            fRasterMultisampleSupport = ctxInfo.hasExtension("GL_EXT_raster_multisample");
+            if (fRasterMultisampleSupport) {
+                GR_GL_GetIntegerv(gli, GR_GL_MAX_RASTER_SAMPLES, &fMaxRasterSamples);
+            }
         }
     }