Context

AnimationSheetBuilder is a testing utility
class that records frames of an animating widget,
and later composes the frames into a single
animation sheet for golden testing. The old way
of composing involves display to list the images
into a table-like widget, adjusting the testing
surface with sheetSize, and capturing the table
widget for comparison. A new way, collate, has
been added that directly puts the frames together
into an image for comparison, which requires less
boilerplate code and outputs a smaller image without
compromise in quality. APIs for the old way are thus
deprecated.

The reason why collate outputs a smaller image,
is because the old way captures on a testing surface
with pixel ratio 3.0, which means it uses a 3x3 pixel
block of the exactly same color to represent 1 actual
pixel, making the image 9 times as large as necessary
(before PNG compression).

Description of change

The following changes have been made to the
AnimationSheetBuilder class: