`DynamicFontTableProvider` is not `Send + Sync`

[PanieriLorenzo]

DynamicFontTableProvider is not Send + Sync despite all current implementations of FontTableProvider being Send + Sync. This limits the user of the crate as Font<DynamicFontTableProvider> is also not Send + Sync so it can't easily be used in multi-threaded applications, without an unsafe impl. Because it is not Send, it cannot be made Sync with a Mutex either.

Suggested Fix

This is the current implementation:

pub struct DynamicFontTableProvider<'a> {
    sfnt_version: u32,
    provider: Box<dyn FontTableProvider + 'a>,
}

And this is my suggested fix:

pub struct DynamicFontTableProvider<'a> {
    sfnt_version: u32,
    provider: Box<dyn FontTableProvider + Send + Sync + 'a>,
}

Potential Downsides

Perhaps in the future there could be a FontTableProvider that isn't Send + Sync, in which case the DynamicFontTableProvider can't be used. Perhaps functions returning DynamicFontTableProvider, like FontData::table_provider could instead return a Send + Sync wrapper.

[wezm]

The suggested fix seems fine if you wanted to open a PR.