add SkPixmap and external locking to bitmaps

include/core/SkImageInfo.h

Index: include/core/SkImageInfo.h
diff --git a/include/core/SkImageInfo.h b/include/core/SkImageInfo.h
index 01318fd1aa95e5b64f445bcf952a2b8adadc4a88..daa50dcdb22a28fae6d4182dbe595471a0786e44 100644
--- a/include/core/SkImageInfo.h
+++ b/include/core/SkImageInfo.h
@@ -111,6 +111,17 @@ static inline bool SkColorTypeIsValid(unsigned value) {
     return value <= kLastEnum_SkColorType;
 }
 
+static inline size_t SkColorTypeComputeOffset(SkColorType ct, int x, int y, size_t rowBytes) {
+    int shift = 0;
+    switch (SkColorTypeBytesPerPixel(ct)) {
+        case 4: shift = 2; break;
+        case 2: shift = 1; break;
+        case 1: shift = 0; break;
+        default: return 0;

[scroggo, 2015/05/19 20:11:42]

Should this SkASSERT(false)?

[reed1, 2015/05/21 20:02:00]

kUnknown_SkColorType correctly should return 0

[scroggo, 2015/05/21 20:20:55]

Right, but it seems like that is correct but not interesting. If someone called
this with kUnknown_SkColorType, aren't they going to run into trouble later, if
they were going to do anything with the information? 

[reed1, 2015/05/21 21:19:02]

This is calling SkColorTypeBytesPerPixels, which already behaves this same-way :
it returns a 0 for these configs. If we want to make this sort of logical
change, I'd like to make it across the board (in this and the other method) to
keep them mutually consistent. If we choose to do that, I'd prefer to do it in a
separate CL.

+    }
+    return y * rowBytes + (x << shift);
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 /**
@@ -247,6 +258,12 @@ public:
         return (size_t)this->minRowBytes64();
     }
 
+    size_t computeOffset(int x, int y, size_t rowBytes) const {
+        SkASSERT((unsigned)x < (unsigned)fWidth);
+        SkASSERT((unsigned)y < (unsigned)fHeight);
+        return SkColorTypeComputeOffset(fColorType, x, y, rowBytes);
+    }
+
     bool operator==(const SkImageInfo& other) const {
         return 0 == memcmp(this, &other, sizeof(other));
     }