Add Sdl2 backend

[TonalidadeHidrica]

This is the Take 3 of providing SDL2 backend for cornod.

I didn't use OpenGL

When it comes to using SDL2, there are a big choice: to use OpenGL or not to use. In the prior art,

• [WIP] SDL2 backend #996 authored by @jonas-schievink does not rely on OpenGL (consequently not on cornod_gfx etc) but used only the feature of "pure SDL2" (including SDL2_gfx).
• SDL2 backend and an example using it and gfx in all_sdl2_gfx_rs.rs #1055 and SDL2 backend and an example using it and gfx in all_sdl2_gfx_rs.rs #1084 authored by @elrnv does use OpenGL.

In this PR, I took the course of not using it, because I wanted to keep the dependencies minimal.

I checked that all the features of DemoApp works, but feel free to pointing out anything I am missing.

Credits go to:

• @elrnv for event conversion code. I copy-pasted all your work (but included as the author when committing that part!).
• @jonas-schievink for how to render text. Although I refined the implementation and adopt newer APIs, the work could have not been done without your code.

About Rust version

Since I could not find the information of MSRV, I developed conrod_sdl2 crate with 1.57.0 and Rust 2021 Edition. If I should use older ones, I would update the code.

About the addition to the dependency

I added thiserror to minimize the boilerplates on error handling, and smallvec for a iterator-friendly container of up-to-two events. If this matters, let me know and I'll think the other way.

Notes on graphics

When you run all_sdl2, you will see the thin diagonal lines as well as horizontal and vertical noize-like lines. These are probably due to overlapping extremely-thin transparent triangles. Unlike GL families, triangles in SDL seems not be cut out for forming complex polygons from many triangles. I think this is inevitable, as long as we use pure SDL2.

[elrnv]

Nice work! I'm happy you found the old PRs useful, and thank you for the mention :)