Separate interface definition and implementation of Snapshot

The Snapshot class is getting complicated enough that its implementation
should now live in a separate .cpp file. This will become particularly
useful when support for clip regions and paths will be added later on.

Change-Id: I050fac5683a9f7a0ff2f7a6beec3dd28aa5eb0d8
diff --git a/libs/hwui/Properties.h b/libs/hwui/Properties.h
index 71fb8da..7854729 100644
--- a/libs/hwui/Properties.h
+++ b/libs/hwui/Properties.h
@@ -38,8 +38,8 @@
 #define LAYER_SIZE 64
 
 // Defines the size in bits of the stencil buffer
-// Note: We only want 1 bit, but in practice we'll get 8 bits on all GPUs
-//       for the foreseeable future
+// Note: Only 1 bit is required for clipping but more bits are required
+// to properly implement the winding fill rule when rasterizing paths
 #define STENCIL_BUFFER_SIZE 0
 
 /**