HWUI: set correct sampler for external texture in shaders

Test: hwuimacro hwbitmapcompositeshader
bug:30999911
Change-Id: Ic63f7109a4a7069b62c0b21efae2d4ba7e6d64be
diff --git a/libs/hwui/SkiaShader.cpp b/libs/hwui/SkiaShader.cpp
index 34e6a06..a9de869 100644
--- a/libs/hwui/SkiaShader.cpp
+++ b/libs/hwui/SkiaShader.cpp
@@ -218,6 +218,7 @@
     const float height = outData->bitmapTexture->height();
 
     description->hasBitmap = true;
+    description->isShaderBitmapExternal = hwuiBitmap->isHardware();
     // gralloc doesn't support non-clamp modes
     if (hwuiBitmap->isHardware() || (!caches.extensions().hasNPot()
             && (!isPowerOfTwo(width) || !isPowerOfTwo(height))