[appveyor] Update VS version to 2017

It seems like VS 2015 may be generating bad code (see binji#20). Let's try
using 2017 instead.

Also, include SDL2.dll with the Windows bundles.

Add missing + in `add sp,%hhd`

Fix high DPI rendering on MacOS

According to the SDL2 docs, `SDL_GetWindowSize` returns the client
window size in screen coordinates, `SDL_GL_GetDrawableSize` returns the
size in pixels.

This change makes sure the `glViewport` calls always use pixels.

Update ImGui to v1.65

Fix rewind buffer corruption bug

`RewindState` has a `RewindResult` which it checks when it is going to
truncat the rewind buffer (after a successful rewind.) If the
`RewindResult`'s info pointer is not set, `rewind_truncate_to` assumes
that the rewind was not successful, so it doesn't truncate.

The bug was that a local `RewindResult` was used instead of the one in
`RewindState`, so the info pointer was never set, so the rewind buffer
was never truncated. After a rewind, `rewind_append` would append states
that were out of order. This would break the binary search of the rewind
info.

To find the bug, I ended up adding a sanity check function for the
rewind buffer. So I decided to keep it.

Fix rewind buffer corruption bug

`RewindState` has a `RewindResult` which it checks when it is going to
truncat the rewind buffer (after a successful rewind.) If the
`RewindResult`'s info pointer is not set, `rewind_truncate_to` assumes
that the rewind was not successful, so it doesn't truncate.

The bug was that a local `RewindResult` was used instead of the one in
`RewindState`, so the info pointer was never set, so the rewind buffer
was never truncated. After a rewind, `rewind_append` would append states
that were out of order. This would break the binary search of the rewind
info.

To find the bug, I ended up adding a sanity check function for the
rewind buffer. So I decided to keep it.

Render using OpenGL instead of SDL renderer