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

include/gpu/GrContext.h

 /*
  * Copyright 2010 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrContext_DEFINED
 #define GrContext_DEFINED
 
@@ skipping 416 matching lines @@
 
     ///////////////////////////////////////////////////////////////////////////
     // Draws
 
     /**
      * Clear the entire or rect of the render target, ignoring any clips.
      * @param rect  the rect to clear or the whole thing if rect is NULL.
      * @param color the color to clear to.
      * @param canIgnoreRect allows partial clears to be converted to whole
      *                      clears on platforms for which that is cheap
-     * @param target if non-NULL, the render target to clear otherwise clear
-     *               the current render target
+     * @param target The render target to clear.
      */
-    void clear(const SkIRect* rect, GrColor color, bool canIgnoreRect,
-               GrRenderTarget* target = NULL);
+    void clear(const SkIRect* rect, GrColor color, bool canIgnoreRect, GrRenderTarget* target);
 
     /**
      *  Draw everywhere (respecting the clip) with the paint.
      */
     void drawPaint(const GrPaint& paint);
 
     /**
      *  Draw the rect using a paint.
      *  @param paint        describes how to color pixels.
      *  @param strokeInfo   the stroke information (width, join, cap), and.
@@ skipping 127 matching lines @@
         /** Any surface writes should be flushed to the backend 3D API after the surface operation
             is complete */
         kFlushWrites_PixelOp = 0x2,
         /** The src for write or dst read is unpremultiplied. This is only respected if both the
             config src and dst configs are an RGBA/BGRA 8888 format. */
         kUnpremul_PixelOpsFlag  = 0x4,
     };
 
     /**
      * Reads a rectangle of pixels from a render target.
-     * @param target        the render target to read from. NULL means the current render target.
+     * @param target        the render target to read from.
      * @param left          left edge of the rectangle to read (inclusive)
      * @param top           top edge of the rectangle to read (inclusive)
      * @param width         width of rectangle to read in pixels.
      * @param height        height of rectangle to read in pixels.
      * @param config        the pixel config of the destination buffer
      * @param buffer        memory to read the rectangle into.
      * @param rowBytes      number of bytes bewtween consecutive rows. Zero means rows are tightly
      *                      packed.
      * @param pixelOpsFlags see PixelOpsFlags enum above.
      *
@@ skipping 413 matching lines @@
     /**
      *  This callback allows the resource cache to callback into the GrContext
      *  when the cache is still overbudget after a purge.
      */
     static bool OverbudgetCB(void* data);
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif