TMX format: Support embedded images on the tileset

[bjorn]

Previously, embedded images were only supported per-tile. Due to the recent change to use sub-rects rather than physically cutting up the tileset, when you called Tileset.loadFromImage(image), the entire tileset image would get duplicated for each tile.

Now the image is embedded once at the tileset level instead.

Also, the Tileset.margin and Tileset.tileSpacing properties are now writable in the scripting API.

Addresses part of #3630.

[eishiya]

The documentation for the XML and JSON formats should probably be amended to mention the possibility of the image source being a base-64 data URL. Currently, this doesn't seem to even be described well for image collection tiles, where this has been a feature for a while. The XML documentation page mentions that an image can have format and data, but links to the description of <data>, which deals solely with tile layer data and not with image data. Is this format/data thing even accurate?

[bjorn]

The XML documentation page mentions that an image can have format and data, but links to the description of <data>, which deals solely with tile layer data and not with image data. Is this format/data thing even accurate?

Yes, this is accurate. Embedded images are not stored as a base-64 data URL, but re-use the "data" element also used by tile layer data. The documentation for the "data" element should be updated to mention this, though.

The JSON format still does not support embedded images. I will look into supporting it and then of course will need to adjust the documentation.

[eishiya]

Gotcha. It'd be good to mention the use for images in the section, even if it's just to say that the data may be binary image data in the case of in elements. Presumably, CSV encoding isn't supported for image data.