Clarify bmp comment about when we support the alpha mask

src/codec/SkBmpCodec.cpp

 /*
  * 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 "SkBmpCodec.h"
 #include "SkBmpMaskCodec.h"
 #include "SkBmpRLECodec.h"
@@ skipping 355 matching lines @@
             SkCodecPrintf("Error: invalid format for bitmap decoding.\n");
             return false;
     }
 
     // Most versions of bmps should be rendered as opaque.  Either they do
     // not have an alpha channel, or they expect the alpha channel to be
     // ignored.  V3+ bmp files introduce an alpha mask and allow the creator
     // of the image to use the alpha channels.  However, many of these images
     // leave the alpha channel blank and expect to be rendered as opaque.  This
     // is the case for almost all V3 images, so we render these as opaque.  For
-    // V4+, we will use the alpha channel, and fix the image later if it turns
-    // out to be fully transparent.
-    // As an exception, V3 bmp-in-ico may use an alpha mask.
+    // V4+ images in kMask mode, we will use the alpha mask.
+    //
+    // skbug.com/4116: We should perhaps also apply the alpha mask in kStandard
+    //                 mode.  We just haven't seen any images that expect this
+    //                 behavior.
+    //
+    // Additionally, V3 bmp-in-ico may use the alpha mask.
     SkAlphaType alphaType = kOpaque_SkAlphaType;
     if ((kInfoV3_BmpHeaderType == headerType && inIco) ||
             kInfoV4_BmpHeaderType == headerType ||
             kInfoV5_BmpHeaderType == headerType) {
         // Header types are matched based on size.  If the header is
         // V3+, we are guaranteed to be able to read at least this size.
         SkASSERT(infoBytesRemaining > 52);
         inputMasks.alpha = get_int(iBuffer.get(), 48);
         if (inputMasks.alpha != 0) {
             alphaType = kUnpremul_SkAlphaType;
@@ skipping 169 matching lines @@
     return prepareToDecode(dstInfo, options, inputColorPtr, inputColorCount);
 }
 
 int SkBmpCodec::onGetScanlines(void* dst, int count, size_t rowBytes) {
     // Create a new image info representing the portion of the image to decode
     SkImageInfo rowInfo = this->dstInfo().makeWH(this->dstInfo().width(), count);
 
     // Decode the requested rows
     return this->decodeRows(rowInfo, dst, rowBytes, this->options());
 }