Don't allow renderTarget==NULL to GrContext::clear() and friends.

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 1383 matching lines @@
     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) {
+    SkASSERT(renderTarget);
     if (!this->caps()->discardRenderTargetSupport()) {
         return;
     }
-    if (NULL == renderTarget) {
-        renderTarget = this->drawState()->getRenderTarget();
-        if (NULL == renderTarget) {
-            return;
-        }
-    }
 
     GrGLRenderTarget* glRT = static_cast<GrGLRenderTarget*>(renderTarget);
     if (renderTarget->getUniqueID() != fHWBoundRenderTargetUniqueID) {
         fHWBoundRenderTargetUniqueID = SK_InvalidUniqueID;
         fGPUStats.incRenderTargetBinds();
         GL_CALL(BindFramebuffer(GR_GL_FRAMEBUFFER, glRT->renderFBOID()));
     }
     switch (this->glCaps().invalidateFBType()) {
         case GrGLCaps::kNone_InvalidateFBType:
             SkFAIL("Should never get here.");
@@ skipping 1143 matching lines @@
             this->setVertexArrayID(gpu, 0);
         }
         int attrCount = gpu->glCaps().maxVertexAttributes();
         if (fDefaultVertexArrayAttribState.count() != attrCount) {
             fDefaultVertexArrayAttribState.resize(attrCount);
         }
         attribState = &fDefaultVertexArrayAttribState;
     }
     return attribState;
 }