Add discard API to SkCanvas, plumb it to glDiscardFramebuffer()

src/gpu/gl/GrGpuGL.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #include "GrGpuGL.h"
 #include "GrGLStencilBuffer.h"
@@ skipping 1277 matching lines @@
     r = GrColorUnpackR(color) * scaleRGB;
     g = GrColorUnpackG(color) * scaleRGB;
     b = GrColorUnpackB(color) * scaleRGB;
 
     GL_CALL(ColorMask(GR_GL_TRUE, GR_GL_TRUE, GR_GL_TRUE, GR_GL_TRUE));
     fHWWriteToColor = kYes_TriState;
     GL_CALL(ClearColor(r, g, b, a));
     GL_CALL(Clear(GR_GL_COLOR_BUFFER_BIT));
 }
 
+void GrGpuGL::discard(GrRenderTarget* renderTarget) {
+    if (NULL == renderTarget) {
+        renderTarget = this->drawState()->getRenderTarget();
+        if (NULL == renderTarget) {
+            return;
+        }
+    }
+
+    GrGLRenderTarget* glRT = static_cast<GrGLRenderTarget*>(renderTarget);
+    if (renderTarget != fHWBoundRenderTarget) {
+        fHWBoundRenderTarget = NULL;
+        GL_CALL(BindFramebuffer(GR_GL_FRAMEBUFFER, glRT->renderFBOID()));
+    }
+    GrGLenum attachments[] = { GR_GL_COLOR };
+    GL_CALL(DiscardFramebuffer(GR_GL_FRAMEBUFFER, SK_ARRAY_COUNT(attachments), attachments));
+    renderTarget->flagAsResolved();
+}
+
+
 void GrGpuGL::clearStencil() {
     if (NULL == this->getDrawState().getRenderTarget()) {
         return;
     }
 
     this->flushRenderTarget(&SkIRect::EmptyIRect());
 
     GrAutoTRestore<ScissorState> asr(&fScissorState);
     fScissorState.fEnabled = false;
     this->flushScissor();
@@ skipping 1481 matching lines @@
             this->setVertexArrayID(gpu, 0);
         }
         int attrCount = gpu->glCaps().maxVertexAttributes();
         if (fDefaultVertexArrayAttribState.count() != attrCount) {
             fDefaultVertexArrayAttribState.resize(attrCount);
         }
         attribState = &fDefaultVertexArrayAttribState;
     }
     return attribState;
 }