Before this fix, when a sprite's color mode was set to Grayscale and
a tileset was exported, the resulting sheet was full of broken
tile images due to a misinterpretation of the pixel format in
the samples.
An unnecessary conversion of the original sprite was also observed
during the "Export Sprite Sheet" command. Specifically,
in the DocExporter::renderTexture function (fix#5088).
New json.decode(jsonText) and json.encode(luaTable) functions.
In this way we don't depend on third-party libraries to decode/encode
JSON text which is a quite common task (in tests and export scripts).
Now the equivalent was "{tag}_{frame1}.png", but it looks like a
regression reported here https://community.aseprite.org/t/17253 were
it was possible to just specify the frame number as in "{tag}_1.png"