Fix compilation on ARM/AARCH64

[MagaTailor]

error[E0308]: mismatched types
    --> src/renderer/mod.rs:1042:41
     |
1042 |             gl::ShaderSource(shader, 1, &(source.as_ptr() as *const i8), len.as_ptr());
     |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected u8, found i8
     |
     = help: here are some functions which might fulfill your needs:
 - .offset(...)
 - .wrapping_offset(...)

error[E0308]: mismatched types
   --> src/tty.rs:174:43
    |
174 |         libc::getpwuid_r(uid, &mut entry, buf.as_mut_ptr(), buf.len(), &mut res)
    |                                           ^^^^^^^^^^^^^^^^ expected u8, found i8
    |
    = help: here are some functions which might fulfill your needs:
 - .offset(...)
 - .wrapping_offset(...)

Not exactly cross-platform, eh?

rust-lang/rust#29867

[jwilm]

Thanks for the fixes! Any thoughts about how we might keep ARM builds from regressing?

[MagaTailor]

As we're talking about fallout from that particular breaking change, I'd say avoid explicit stuff like Vec<*const i8> or as_ptr() as *const i8, etc.

[jwilm]

Er, I was thinking more from a CI perspective. Your advice it noted, though.

[MagaTailor]

Oh, in that case cross-compile for ARM obviously :) **edit:** Updated the original comment to include the errors.

[jwilm]

Thanks for the PR. I'll file an issue about getting CI added.

[MagaTailor]

Great, please consider adding GLES (or at least OpenGL 2.0) support to make alacritty runnable on a typical ARM system. Thanks!