x11: Overhaul XIM code (#451)

Fixes #195
Fixes #277
Fixes #455

* Read `XMODIFIERS` explicitly/directly instead of calling `XSetLocaleModifiers` with an
empty string. This is useful for debugging purposes, and more clear to read and handle.
* Fallback to local input method if the one specified in `XMODIFIERS` is later closed on the
server end (i.e. if ibus/fcitx is terminated). Previously, that would cause the event loop
to freeze and usually also segfault.
* If using the fallback input method, respond to the `XMODIFIERS` input method later
becoming available. This means that the input method restarting is handled, and that even if
the program was started while ibus/fcitx/etc. was unavailable, it will start using it as
soon as it becomes available.
* Only one input method is opened for the whole event loop, with each window having its own
input context.
* IME works completely out of the box now, no longer requiring application developers to
call `setlocale` or `XSetLocaleModifiers`.
* Detailed error messages are provided if no input method could be opened. However, no
information is provided to the user if their intended `XMODIFIERS` input method failed to
open but the fallbacks (which will ostensibly always succeed) succeeded; in my opinion, this
is something that is best filled by adding a logging feature to winit.

src/platform/linux/x11/ime/inner.rs

@@ -0,0 +1,75 @@
+use std::mem;
+use std::ptr;
+use std::sync::Arc;
+use std::collections::HashMap;
+
+use super::{ffi, XConnection, XError};
+
+use super::input_method::PotentialInputMethods;
+use super::context::ImeContext;
+
+pub unsafe fn close_im(xconn: &Arc<XConnection>, im: ffi::XIM) -> Result<(), XError> {
+    (xconn.xlib.XCloseIM)(im);
+    xconn.check_errors()
+}
+
+pub unsafe fn destroy_ic(xconn: &Arc<XConnection>, ic: ffi::XIC) -> Result<(), XError> {
+    (xconn.xlib.XDestroyIC)(ic);
+    xconn.check_errors()
+}
+
+pub struct ImeInner {
+    pub xconn: Arc<XConnection>,
+    // WARNING: this is initially null!
+    pub im: ffi::XIM,
+    pub potential_input_methods: PotentialInputMethods,
+    pub contexts: HashMap<ffi::Window, Option<ImeContext>>,
+    // WARNING: this is initially zeroed!
+    pub destroy_callback: ffi::XIMCallback,
+    // Indicates whether or not the the input method was destroyed on the server end
+    // (i.e. if ibus/fcitx/etc. was terminated/restarted)
+    pub is_destroyed: bool,
+    pub is_fallback: bool,
+}
+
+impl ImeInner {
+    pub fn new(
+        xconn: Arc<XConnection>,
+        potential_input_methods: PotentialInputMethods,
+    ) -> Self {
+        ImeInner {
+            xconn,
+            im: ptr::null_mut(),
+            potential_input_methods,
+            contexts: HashMap::new(),
+            destroy_callback: unsafe { mem::zeroed() },
+            is_destroyed: false,
+            is_fallback: false,
+        }
+    }
+
+    pub unsafe fn close_im_if_necessary(&self) -> Result<bool, XError> {
+        if !self.is_destroyed {
+            close_im(&self.xconn, self.im).map(|_| true)
+        } else {
+            Ok(false)
+        }
+    }
+
+    pub unsafe fn destroy_ic_if_necessary(&self, ic: ffi::XIC) -> Result<bool, XError> {
+        if !self.is_destroyed {
+            destroy_ic(&self.xconn, ic).map(|_| true)
+        } else {
+            Ok(false)
+        }
+    }
+
+    pub unsafe fn destroy_all_contexts_if_necessary(&self) -> Result<bool, XError> {
+        for context in self.contexts.values() {
+            if let &Some(ref context) = context {
+                self.destroy_ic_if_necessary(context.ic)?;
+            }
+        }
+        Ok(!self.is_destroyed)
+    }
+}