Make size/position types generic over pixel type (#1277)

* Begin implementing DPI generics

* Fix multithreaded example

* Format

* Fix serde test

* hopefully fix most of the errors

* Fix dpi module errors

* More error fixings

* Format

* fix macos errors

* Another error pass

* Replace bad type signatures

* more fixins

src/platform_impl/ios/ffi.rs

@@ -43,7 +43,7 @@ pub struct CGSize {
 }
 
 impl CGSize {
-    pub fn new(size: LogicalSize) -> CGSize {
+    pub fn new(size: LogicalSize<f64>) -> CGSize {
         CGSize {
             width: size.width as _,
             height: size.height as _,