Make SkBitmap::CopyTo respect requested dst color type when bitmap is texture backed.

BUG=chromium:550559
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1576983002

Review URL: https://codereview.chromium.org/1576983002
diff --git a/src/core/SkBitmap.cpp b/src/core/SkBitmap.cpp
index c66a5b2..bdf1daa 100644
--- a/src/core/SkBitmap.cpp
+++ b/src/core/SkBitmap.cpp
@@ -828,7 +828,7 @@
         SkIRect subset;
         subset.setXYWH(fPixelRefOrigin.fX, fPixelRefOrigin.fY,
                        fInfo.width(), fInfo.height());
-        if (fPixelRef->readPixels(&tmpSrc, &subset)) {
+        if (fPixelRef->readPixels(&tmpSrc, dstColorType, &subset)) {
             if (fPixelRef->info().alphaType() == kUnpremul_SkAlphaType) {
                 // FIXME: The only meaningful implementation of readPixels
                 // (GrPixelRef) assumes premultiplied pixels.