Capture fractional pixel coordinates in the bounds of raster cache bi…

…tmaps (flutter#3974)

The bitmap used to hold a prerolled image in the raster cache must have an
integer width/height in pixels.  However, the picture used to draw the image
may have a non-integer width/height in physical coordinates and thus not
completely fill the bitmap.  The compositor should only select the subset
of the bitmap that reflects the portion filled by the rendered picture.

flow/raster_cache.cc

@@ -79,12 +79,12 @@ RasterCacheResult RasterizePicture(SkPicture* picture,
 
   const SkRect logical_rect = picture->cullRect();
   const SkRect physical_rect =
-      SkRect::MakeWH(std::ceil(std::fabs(logical_rect.width() * scale.x())),
-                     std::ceil(std::fabs(logical_rect.height() * scale.y())));
+      SkRect::MakeWH(std::fabs(logical_rect.width() * scale.x()),
+                     std::fabs(logical_rect.height() * scale.y()));
 
   const SkImageInfo image_info =
-      SkImageInfo::MakeN32Premul(physical_rect.width(),      // physical width
-                                 physical_rect.height(),     // physical height
+      SkImageInfo::MakeN32Premul(std::ceil(physical_rect.width()),      // physical width
+                                 std::ceil(physical_rect.height()),     // physical height
                                  sk_ref_sp(dst_color_space)  // colorspace
       );