Use ControlFlow:Poll in event loop

ControlFlow::Wait is more efficient but doesn't appear to currently work
well on all platforms. Removed entirely from the earlier examples to
keep them as simple as possible, but noted `ControlFlow::Wait` in the
last example

Closes imgui-rs#542

imgui-glow-renderer/examples/01_basic.rs

@@ -29,7 +29,6 @@ fn main() {
 
     // Standard winit event loop
     event_loop.run(move |event, _, control_flow| {
-        *control_flow = glutin::event_loop::ControlFlow::Wait;
         match event {
             glutin::event::Event::NewEvents(_) => {
                 let now = Instant::now();

imgui-glow-renderer/examples/02_triangle.rs

@@ -17,7 +17,6 @@ fn main() {
 
     let mut last_frame = Instant::now();
     event_loop.run(move |event, _, control_flow| {
-        *control_flow = glutin::event_loop::ControlFlow::Wait;
         match event {
             glutin::event::Event::NewEvents(_) => {
                 let now = Instant::now();

imgui-glow-renderer/examples/03_triangle_gles.rs

@@ -32,7 +32,6 @@ fn main() {
 
     let mut last_frame = Instant::now();
     event_loop.run(move |event, _, control_flow| {
-        *control_flow = glutin::event_loop::ControlFlow::Wait;
         match event {
             glutin::event::Event::NewEvents(_) => {
                 let now = Instant::now();

imgui-glow-renderer/examples/04_custom_textures.rs

@@ -36,7 +36,11 @@ fn main() {
 
     let mut last_frame = Instant::now();
     event_loop.run(move |event, _, control_flow| {
-        *control_flow = glutin::event_loop::ControlFlow::Wait;
+        // Note we can potentially make the loop more efficient by
+        // changing the `Poll` (default) value to `ControlFlow::Poll`
+        // but be careful to test on all target platforms!
+        *control_flow = glutin::event_loop::ControlFlow::Poll;
+
         match event {
             glutin::event::Event::NewEvents(_) => {
                 let now = Instant::now();