Implement public API for high-DPI (#319)

* Implement public API for high-DPI #105

* Recover get_inner_size_points and get_inner_size_pixels and change their implementation assuming get_inner_size() returns size in pixels

* Update changelog for high-DPI changes

src/platform/emscripten/mod.rs

@@ -42,6 +42,11 @@ impl MonitorId {
     pub fn get_dimensions(&self) -> (u32, u32) {
         (0, 0)
     }
+
+    #[inline]
+    pub fn get_hidpi_factor(&self) -> f32 {
+        1.0
+    }
 }
 
 // Used to assign a callback to emscripten main loop