add MakeS32 helper to SkImageInfo, fix named-gamma constructor bug

src/core/SkImageInfo.cpp

 /*
  * Copyright 2010 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkImageInfo.h"
 #include "SkImageInfoPriv.h"
 #include "SkReadBuffer.h"
@@ skipping 17 matching lines @@
 
 static bool color_type_is_valid(SkColorType colorType) {
     return (colorType >= 0) && (colorType <= kLastEnum_SkColorType);
 }
 
 SkImageInfo SkImageInfo::Make(int width, int height, SkColorType ct, SkAlphaType at,
                               sk_sp<SkColorSpace> cs) {
     return SkImageInfo(width, height, ct, at, SkDefaultColorProfile(), std::move(cs));
 }
 
+SkImageInfo SkImageInfo::MakeS32(int width, int height, SkAlphaType at) {
+    return SkImageInfo(width, height, kN32_SkColorType, at, kSRGB_SkColorProfileType,
+                       SkColorSpace::NewNamed(SkColorSpace::kSRGB_Named));
+}
+
 void SkImageInfo::unflatten(SkReadBuffer& buffer) {
     fWidth = buffer.read32();
     fHeight = buffer.read32();
 
     uint32_t packed = buffer.read32();
     SkASSERT(0 == (packed >> 24));
     fProfileType = (SkColorProfileType)((packed >> 16) & 0xFF);
     fAlphaType = (SkAlphaType)((packed >> 8) & 0xFF);
     fColorType = (SkColorType)((packed >> 0) & 0xFF);
     buffer.validate(profile_type_is_valid(fProfileType) &&
@@ skipping 80 matching lines @@
     }
     // here x,y are either 0 or negative
     fPixels = ((char*)fPixels - y * fRowBytes - x * fInfo.bytesPerPixel());
     // the intersect may have shrunk info's logical size
     fInfo = fInfo.makeWH(srcR.width(), srcR.height());
     fX = srcR.x();
     fY = srcR.y();
 
     return true;
 }