Improve Simulator CPU Usage up to 50%.

[grantshandy]

• Passes tests and documentation builds
• Passes just build
• Passes rustfmt
• No changes to public API.
• Closes Bad performance when animating #42

Description

This PR includes some minor changes to OutputSettings, Window, and SdlWindow which greatly improve the performance of the embedded graphics simulator.

Changes

Create TextureCreator and Texture at the initialization of SdlWindow, not on each frame.

@rfuest mentioned that the texture creator and texture had to be created on each frame because of a lifetime issue. I encountered this "self-referencing lifetime memory issue" too, but was able to overcome it by using the proc macro ouroboros on a new private struct SdlWindowTexture.

Limit FPS to 60Hz default.

One of the main factors that led to simulator programs usually taking ~100% of a CPU core was letting the program "run wild in the hot loop" by default. Now the program puts the thread to sleep if the draw took less than the desired fps. I set the default FPS limit as 60Hz, but this can be changed in OutputSettings. I imagine most programs written with the embedded-graphics framework in mind aren't FPS-dependent, but if they are I'm sure they can set the max fps to much higher in order to eliminate this effect.

Performance Improvements

I've found that with the input-handling example, the changes in this PR improve my CPU usage by ~50%!

Here are the results of ps -p <pid> -o %cpu,%mem,cmd on the master branch:

%CPU %MEM CMD
99.5  0.8 target/release/examples/input-handling

And on my fork:

%CPU %MEM CMD
49.7  0.8 target/release/examples/input-handling

[jamwaffles]

@rfuest any comments before I merge this?

[rfuest]

No, LGTM. The only thing that is missing is a changelog entry for the new setting and the changed behavior of update.

[jamwaffles]

Ah of course. @grantshandy under the Unreleased section in CHANGELOG.md, could you please add the following (or similar if you want to word it differently):

### Added

- [#45](https://github.com/embedded-graphics/simulator/pull/45) Added `OutputSettingsBuilder::max_fps` to set the maximum FPS of the simulator.
  
### Changed

- [#45](https://github.com/embedded-graphics/simulator/pull/45) Limit simulator to 60FPS by default.

[bugadani]

This sleep assumes my processing takes 0 time. Can you store the time after the thread wakes up, then use that as a reference instead of grabbing it at the start of update, please?

Also, in that case, my processing may take a long time and it will be possible for the sleep time to be "negative", in which case what happens is currently rust version dependent, as stated here. Could you please turn this into a checked subtraction?

[grantshandy]

Thanks for noticing this! I'll apply these suggestions and add the section in CHANGELOG.md that @jamwaffles suggested this afternoon. 👍🏽

[grantshandy]

thread::sleep(
    (self.frame_start + self.desired_loop_duration)
        .saturating_duration_since(Instant::now()),
);

self.frame_start = Instant::now();

If your frame takes a long time and becomes "negative", saturating_duration_since will return zero, which I think will fix the issue.

[bugadani]

Thanks for decoding my awkward comment perfectly :)

[grantshandy]

I made a mistake naming this commit (CONTRIBUTING.md, not CHANGELOG.md), but it should address the issues @rfuest and @bugadani brought up.

[jamwaffles]

Looks good. Thanks very much!