Add emoji fonts support

[cppHusky]

I found emoji fonts cannot be loaded by freetype-rs correctly, for example, when I try to load Noto Color Emoji,

$ cargo r --example single_glyph /usr/share/fonts/noto/NotoColorEmoji.ttf 🙂
warning: creating a mutable reference to mutable static is discouraged
  --> src/library.rs:53:48
   |
53 |         let err = unsafe { ffi::FT_New_Library(&mut MEMORY, &mut raw) };
   |                                                ^^^^^^^^^^^ mutable reference to mutable static
   |
   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/static-mut-references.html>
   = note: mutable references to mutable statics are dangerous; it's undefined behavior if any other pointer to the static is used or if any other reference is created for the static while the mutable reference lives
   = note: `#[warn(static_mut_refs)]` on by default
help: use `&raw mut` instead to create a raw pointer
   |
53 |         let err = unsafe { ffi::FT_New_Library(&raw mut MEMORY, &mut raw) };
   |                                                 +++

warning: `freetype-rs` (lib) generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.03s
     Running `target/debug/examples/single_glyph /usr/share/fonts/noto/NotoColorEmoji.ttf '🙂'`

thread 'main' panicked at examples/single_glyph.rs:40:43:
called `Result::unwrap()` on an `Err` value: InvalidPixelSize
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/17067e9ac6d7ecb70e50f92c1944e545188d2359/library/std/src/panicking.rs:697:5
   1: core::panicking::panic_fmt
             at /rustc/17067e9ac6d7ecb70e50f92c1944e545188d2359/library/core/src/panicking.rs:75:14
   2: core::result::unwrap_failed
             at /rustc/17067e9ac6d7ecb70e50f92c1944e545188d2359/library/core/src/result.rs:1704:5
   3: core::result::Result<T,E>::unwrap
             at /rustc/17067e9ac6d7ecb70e50f92c1944e545188d2359/library/core/src/result.rs:1109:23
   4: single_glyph::main
             at ./examples/single_glyph.rs:40:5
   5: core::ops::function::FnOnce::call_once
             at /rustc/17067e9ac6d7ecb70e50f92c1944e545188d2359/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

However, if I load Noto Sans it works well. (but without glyph in this font it displays a tofu)

$ cargo r --example single_glyph /usr/share/fonts/noto/NotoSans-Regular.ttf 🙂
warning: creating a mutable reference to mutable static is discouraged
  --> src/library.rs:53:48
   |
53 |         let err = unsafe { ffi::FT_New_Library(&mut MEMORY, &mut raw) };
   |                                                ^^^^^^^^^^^ mutable reference to mutable static
   |
   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/static-mut-references.html>
   = note: mutable references to mutable statics are dangerous; it's undefined behavior if any other pointer to the static is used or if any other reference is created for the static while the mutable reference lives
   = note: `#[warn(static_mut_refs)]` on by default
help: use `&raw mut` instead to create a raw pointer
   |
53 |         let err = unsafe { ffi::FT_New_Library(&raw mut MEMORY, &mut raw) };
   |                                                 +++

warning: `freetype-rs` (lib) generated 1 warning
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.00s
     Running `target/debug/examples/single_glyph /usr/share/fonts/noto/NotoSans-Regular.ttf '🙂'`




  *************
  *+++++++++++*
  *+********++*
  *+*       *+*
  *+*       *+*
  *+*       *+*
  *+*       *+*
  *+*       *+*
  *+*       *+*
  *+*       *+*
  *+*       *+*
  *+*       *+*
  *+*       *+*
  *+*       *+*
  *+*       *+*
  *+*       *+*
  *+*       *+*
  *+*       *+*
  *+*********+*
  *+++++++++++*

Will you consider supporting emoji fonts? Thanks a lot.