Use SkOpts routines in SkSwizzler

src/codec/SkSwizzlerOpts.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+#include "SkOpts.h"
+#include "SkSwizzler.h"
+
+// Unpremultiplied RGBA -> Premultiplied N32
+// This is quite common for PNGs.

[mtklein, 2016/01/08 15:20:19]

My preference would be to plop this function in SkSwizzler.cpp among the rest of
the RowProcs.  Putting this into a header makes me wonder, why is it in a
header, are there two files that need to call this?  The scope of a static
function in a .cpp file, on the other hand, is pretty darn clear.

[msarett, 2016/01/11 20:33:25]

Sounds good.  I was trying to avoid (more) clutter, but it's worth the extra
confusion.

+static SkSwizzler::ResultAlpha opt_swizzle_rgba_to_n32_premul(
+        void* dst, const uint8_t* src, int width, int bpp, int deltaSrc,
+        int offset, const SkPMColor ctable[]) {

[mtklein, 2016/01/08 15:20:20]

Have you ever thought about ditching offset as a parameter to these procs?
You don't ever do anything other than start off with src += offset, right?

[msarett, 2016/01/11 20:33:25]

We have some swizzler routines where bits per pixel is less than 8.

In those cases, the offset could be some number of bits that is not a multiple
of 8.  In those cases, we need to know the offset in the swizzle function.

Alternatively, we could increment src by offset before calling swizzle(), and
then the offset parameter could most of the time be unused.

+
+    // If we are not sampling, deltaSrc should equal bpp.
+    SkASSERT(deltaSrc == bpp);
+
+#ifdef SK_PMCOLOR_IS_RGBA
+    SkOpts::premul_xxxa((uint32_t*) dst, (uint32_t*) (src + offset), width);

[mtklein, 2016/01/08 15:20:20]

probably clearest to use (const uint32_t*)(src + offset) in these calls.

[msarett, 2016/01/11 20:33:25]

Done.

+#else
+    SkOpts::premul_swaprb_xxxa((uint32_t*) dst, (uint32_t*) (src + offset), width);
+#endif
+    // FIXME: Should we continue to support reallyHasAlpha()?

[scroggo, 2016/01/08 16:02:57]

I thought we had already decided not to. 

[mtklein, 2016/01/08 16:23:01]

If we haven't yet, let's decide now to drop it unless later proven necessary.

[msarett, 2016/01/11 20:33:25]

Done.

+    return SkSwizzler::kTransparent_ResultAlpha;
+}

[mtklein, 2016/01/08 15:20:19]

We can also do UPM RGBA -> UPM N32 right?

#ifdef SK_PMCOLOR_IS_RGBA
  memmove(...)
#else
  SkOpts::swaprb_xxxa(...)
#endif

[msarett, 2016/01/11 20:33:25]

Yeah I'm starting with one at a time :)