We should make good use of in-place member initialization. Many
new classes have constructors that effectively do nothing but
default-initialize POD members, and when adding new members,
it is very easy to miss initializing them. With in-place
initialization, the code is more compact, the diffs are nicer,
and it's harder to miss them.
This commit only converts render/ and platform/ to use in-place
member initialization, since there was a bug in CairoRenderer,
but we should convert the entire codebase.