Only clear the part of the texture used by the device (duh!)

R=robertphillips@google.com
Review URL: https://codereview.appspot.com/7205055

git-svn-id: http://skia.googlecode.com/svn/trunk@7393 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/gpu/SkGpuDevice.cpp b/src/gpu/SkGpuDevice.cpp
index 3e20e3b..566c6d7 100644
--- a/src/gpu/SkGpuDevice.cpp
+++ b/src/gpu/SkGpuDevice.cpp
@@ -572,7 +572,8 @@
 
 ///////////////////////////////////////////////////////////////////////////////
 void SkGpuDevice::clear(SkColor color) {
-    fContext->clear(NULL, SkColor2GrColor(color), fRenderTarget);
+    SkIRect rect = SkIRect::MakeWH(this->width(), this->height());
+    fContext->clear(&rect, SkColor2GrColor(color), fRenderTarget);
     fNeedClear = false;
 }