Add GL_CHROMIUM_screen_space_antialiasing to support alternative AA

Source/platform/graphics/gpu/DrawingBuffer.cpp

Index: Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/Source/platform/graphics/gpu/DrawingBuffer.cpp b/Source/platform/graphics/gpu/DrawingBuffer.cpp
index f4d21767f72f093370f435a7a7faef2ea12eb3f1..f8ad4ec371d5488091633183e50593efffe73bd8 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -434,8 +434,12 @@ bool DrawingBuffer::initialize(const IntSize& size)
     if (m_requestedAttributes.antialias && m_multisampleExtensionSupported) {
         m_context->getIntegerv(GL_MAX_SAMPLES_ANGLE, &maxSampleCount);
         m_multisampleMode = ExplicitResolve;
-        if (m_extensionsUtil->supportsExtension("GL_EXT_multisampled_render_to_texture"))
+        if (m_extensionsUtil->supportsExtension("GL_EXT_multisampled_render_to_texture")) {
             m_multisampleMode = ImplicitResolve;
+        }
+        if (m_extensionsUtil->supportsExtension("GL_INTEL_framebuffer_CMAA")) {

[bajones, 2015/08/19 16:26:44]

Nit: This should be an `else if`

[Ken Russell (switch to Gerrit), 2015/08/19 17:56:18]

Or does the Intel hardware supporting INTEL_framebuffer_CMAA support both
extensions? Is CMAA preferred by Intel?

+            m_multisampleMode = INTELCMAAResolve;

[piman, 2015/08/19 16:36:51]

drive-by nit: CMAA is not multisampling, maybe rename m_multisampleMode into
m_antiAliasingMode, and then ImplicitResolve/ExplicitResolve into
MSAAImplicitResolve/MSAAExplicitResolve ?

[Ken Russell (switch to Gerrit), 2015/08/19 17:56:18]

I like Antoine's suggestion. Could you please make that change?

+        }
     }
     m_sampleCount = std::min(4, maxSampleCount);
 
@@ -815,6 +819,11 @@ void DrawingBuffer::commit()
     }
 
     m_context->bindFramebuffer(GL_FRAMEBUFFER, m_fbo);
+    if (m_multisampleMode == INTELCMAAResolve) {
+        // Apply CMAA(Conservative Morphological Anti-Aliasing) algorithm to the color buffer of m_fbo.
+        // Reference: https://software.intel.com/en-us/articles/conservative-morphological-anti-aliasing-cmaa
+        m_context->applyFramebufferAttachmentCMAAINTEL();
+    }
     m_contentsChangeCommitted = true;
 }