Lazy parsing

[c960657]

Currently an instance can only pass one property, so in this case the latter assertion will fail.

fi = FastImage.new(File.join(FixturePath, "animated.gif"))
assert_equal [400, 400], fi.size
assert_equal true, fi.animated

This PR makes adds lazy parsing. With the new lazy option, parsing isn't initiated from the constructor, and new properties are parsed on demand.

In particular, File.size? is now only invoked if content_length is requested. This is a small performance improvement.

Instead of returning the result, the different parsers now populate @size, @orientation etc. whenever it encounters the corresponding data. This allows a parser to populate more than one property in one go.

For backwards compatibility, lazy defaults to false. In a future major release, this could change to true, and type_only/size_only/animated_only can be removed.

[sdsykes]

Note that Array#second doesn't exist outside of active support, have to use size[1]

[sdsykes]

It would seem a bit neater to me to always set @content_length here - and after that do the early return if content length was being asked for

[c960657]

This is intentional.

It was actually what inspired me to make this PR. I had noticed that Fastimage is a bit slower (~10-20% AFAIR) than the GIF reader in Mastodon. I found out that the difference is due to the File.size? call in Fastimage, so it makes sense to skip this unless content_length is explicitly requested.

[sdsykes]

Ah ok!