Implement growing logic for TextAtlas

The `TextAtlas` will have an initial size of `256` (we could make this
configurable) and `try_allocate` will keep track of the glyphs in use
in the current frame, returning `None` when the LRU glyph is in use.

In that case, `TextRenderer::prepare` will return
`PrepareError::AtlasFull` with the `ContentType` of the atlas that is
full. The user of the library can then call `TextAtlas::grow` with the
provided `ContentType` to obtain a bigger atlas (by `256`).

A `TextAtlas::grow` call clears the whole atlas and, as a result, all of
the `prepare` calls need to be repeated in a frame until they all
succeed. Overall, the atlas will rarely need to grow and so the calls
will not need to be repated often.

Finally, the user needs to call `TextAtlas::trim` at the end of the
frame. This allows us to clear the glyphs in use collection in the atlas. Maybe
there is a better way to model this in an API that forces the user to
trim the atlas (e.g. make `trim` return a new type and changing `prepare` and `render` to take that type instead).

src/error.rs

@@ -1,3 +1,4 @@
+use crate::ContentType;
 use std::{
     error::Error,
     fmt::{self, Display, Formatter},
@@ -6,7 +7,7 @@ use std::{
 /// An error that occurred while preparing text for rendering.
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub enum PrepareError {
-    AtlasFull,
+    AtlasFull(ContentType),
 }
 
 impl Display for PrepareError {