store SkAlphaType inside SkBitmap, on road to support unpremul

src/core/SkBitmapScaler.cpp

 #include "SkBitmapScaler.h"
 #include "SkBitmapFilter.h"
 #include "SkRect.h"
 #include "SkTArray.h"
 #include "SkErrorInternals.h"
 #include "SkConvolver.h"
 
 // SkResizeFilter ----------------------------------------------------------------
 
 // Encapsulates computation and storage of the filters required for one complete
@@ skipping 269 matching lines @@
 
     // Get a source bitmap encompassing this touched area. We construct the
     // offsets and row strides such that it looks like a new bitmap, while
     // referring to the old data.
     const unsigned char* sourceSubset =
         reinterpret_cast<const unsigned char*>(source.getPixels());
 
     // Convolve into the result.
     SkBitmap result;
     result.setConfig(SkBitmap::kARGB_8888_Config,
-        destSubset.width(), destSubset.height());
+                     destSubset.width(), destSubset.height(), 0,
+                     source.alphaType());
     result.allocPixels(allocator, NULL);
     if (!result.readyToDraw()) {
         return false;
     }
 
     BGRAConvolve2D(sourceSubset, static_cast<int>(source.rowBytes()),
         !source.isOpaque(), filter.xFilter(), filter.yFilter(),
         static_cast<int>(result.rowBytes()),
         static_cast<unsigned char*>(result.getPixels()),
         convolveProcs, true);
 
-    // Preserve the "opaque" flag for use as an optimization later.
-    result.setIsOpaque(source.isOpaque());
     *resultPtr = result;
     return true;
 }
 
 // static
 bool SkBitmapScaler::Resize(SkBitmap* resultPtr,
                             const SkBitmap& source,
                             ResizeMethod method,
                             int destWidth, int destHeight,
                             const SkConvolutionProcs& convolveProcs,
                             SkBitmap::Allocator* allocator) {
     SkIRect destSubset = { 0, 0, destWidth, destHeight };
     return Resize(resultPtr, source, method, destWidth, destHeight, destSubset,
                   convolveProcs, allocator);
 }