Emscripten rework (#286)

* partial implementation for emscripten

this pull request contain a partial but working implementation of emscripten backend

some implementations may be controversial.

here some implementation detail:

* cursor state:
  * on grab: emscripten request pointer lock deferred and also set a callback when pointer lock change
             the callback request pointer lock deferred.
  * on hide: `emscripten_hide_mouse` exist but not `emscripten_show_mouse`
             a pull request has been open on october 2016 but never been merged
             so I copied the javascript function and put it in emscripten_asm_const function

* fullscreen: if fullscreen is requested then it request fullscreen deferred and set a callback on fullscreen change
              the callback request fullscreen deferred

* run forever: this method use emscripten main loop to run an infinite loop

* keyboard callback doesn't consume the event. I think it is more apopriate as in desktop environment it is the same, is it ?

* emscripten dir is added in example and contains html pages

Some things that are not implemented:

* lots of events

* min and max dimension can be implemented with a callback that listen to size change and resize if dimension out of bound

* title may be implemented using javascript to change document.title

* Use std::os::raw in the emscripten bindings

* Fix emscripten code

* Update code

* Add CI

* Remove the emscripten-specific examples

* Add some information to the README

.circleci/config.yml

@@ -2,22 +2,52 @@ version: 2
 
 jobs:
 
-  test:
-    working_directory: ~/tgst
+  android-test:
+    working_directory: ~/winit
     docker:
       - image: tomaka/cargo-apk
     steps:
       - checkout
       - restore_cache:
-          key: test-cache-{{ checksum "Cargo.toml" }}
+          key: android-test-cache-{{ checksum "Cargo.toml" }}
       - run: cargo apk build --example window
       - save_cache:
-          key: test-cache-{{ checksum "Cargo.toml" }}
+          key: android-test-cache-{{ checksum "Cargo.toml" }}
           paths:
             - target
 
+  asmjs-test:
+    working_directory: ~/winit
+    docker:
+      - image: tomaka/rustc-emscripten
+    steps:
+      - checkout
+      - restore_cache:
+          key: asmjs-test-cache-{{ checksum "Cargo.toml" }}
+      - run: cargo build --example window --target asmjs-unknown-emscripten
+      - save_cache:
+          key: asmjs-test-cache-{{ checksum "Cargo.toml" }}
+          paths:
+            - target
+
+  wasm-test:
+    working_directory: ~/winit
+    docker:
+      - image: tomaka/rustc-emscripten
+    steps:
+      - checkout
+      - restore_cache:
+          key: wasm-test-cache-{{ checksum "Cargo.toml" }}
+      - run: cargo build --example window --target wasm32-unknown-emscripten
+      - save_cache:
+          key: wasm-test-cache-{{ checksum "Cargo.toml" }}
+          paths:
+            - target
+  
 workflows:
   version: 2
   build-test-and-deploy:
     jobs:
-      - test
+      - android-test
+      - asmjs-test
+      - wasm-test