Simplify SkCodecs' call to SkColorSpaceXform::apply

Most SkCodec subclasses did the following to apply their
SkColorSpaceXform:
  dstFormat = select_xform_format(dstInfo.colorType());
  srcFormat = select_xform_format(<something that doesn't change>);
  xformAlphaType = select_xform_alpha(dstInfo.alphaType(),
                                      this->getInfo().alphaType());
  this->colorXform()->apply(dstFormat, dst, srcFormat, src, width,
                            xformAlphaType);

Consolidate the computation of these parameters into SkCodec and add a
new method to SkCodec that calls apply() with those parameters.

Add a SkColorSpaceXform::ColorFormat to SkCodec. This allows the new
method SkCodec::applyColorXform to supply the ColorFormat.

TBR=reed@google.com
(No change to public API.)

Change-Id: I8ea7ba4c0024be827a9f9359796c778744330f6e
Reviewed-on: https://skia-review.googlesource.com/18523
Reviewed-by: Leon Scroggins <scroggo@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
diff --git a/src/codec/SkWbmpCodec.cpp b/src/codec/SkWbmpCodec.cpp
index d59789b..780ae5e 100644
--- a/src/codec/SkWbmpCodec.cpp
+++ b/src/codec/SkWbmpCodec.cpp
@@ -107,7 +107,9 @@
 }
 
 SkWbmpCodec::SkWbmpCodec(int width, int height, const SkEncodedInfo& info, SkStream* stream)
-    : INHERITED(width, height, info, stream, SkColorSpace::MakeSRGB())
+    // Wbmp does not need a colorXform, so choose an arbitrary srcFormat.
+    : INHERITED(width, height, info, SkColorSpaceXform::ColorFormat(),
+                stream, SkColorSpace::MakeSRGB())
     , fSrcRowBytes(get_src_row_bytes(this->getInfo().width()))
     , fSwizzler(nullptr)
     , fColorTable(nullptr)