Revert of Revert of Extract most of the mutable state of SkShader into a separate Context object.

src/core/SkBitmapProcState.cpp

Index: src/core/SkBitmapProcState.cpp
diff --git a/src/core/SkBitmapProcState.cpp b/src/core/SkBitmapProcState.cpp
index be87d832fdb3fbc30b593a960f0212b714e6ca58..eecfbbcc483c67e42e99c3dfd85bd8b9e2855b7b 100644
--- a/src/core/SkBitmapProcState.cpp
+++ b/src/core/SkBitmapProcState.cpp
@@ -360,17 +360,6 @@
     return true;
 }
 
-void SkBitmapProcState::endContext() {
-    SkDELETE(fBitmapFilter);
-    fBitmapFilter = NULL;
-    fScaledBitmap.reset();
-
-    if (fScaledCacheID) {
-        SkScaledImageCache::Unlock(fScaledCacheID);
-        fScaledCacheID = NULL;
-    }
-}
-
 SkBitmapProcState::~SkBitmapProcState() {
     if (fScaledCacheID) {
         SkScaledImageCache::Unlock(fScaledCacheID);
@@ -399,6 +388,7 @@
     }
     // The above logic should have always assigned fBitmap, but in case it
     // didn't, we check for that now...
+    // TODO(dominikg): Ask humper@ if we can just use an SkASSERT(fBitmap)?
     if (NULL == fBitmap) {
         return false;
     }
@@ -487,6 +477,7 @@
     // shader will perform.
 
     fMatrixProc = this->chooseMatrixProc(trivialMatrix);
+    // TODO(dominikg): SkASSERT(fMatrixProc) instead? chooseMatrixProc never returns NULL.
     if (NULL == fMatrixProc) {
         return false;
     }
@@ -528,6 +519,7 @@
                 fPaintPMColor = SkPreMultiplyColor(paint.getColor());
                 break;
             default:
+                // TODO(dominikg): Should we ever get here? SkASSERT(false) instead?
                 return false;
         }