Initialize render target when window size is not specified on Windows

[Perlmint]

Fix #1426.

As I commented on that issue, When the window size is not specified, WM_RESIZE is not emitted before first WM_PAINT. So, rendering is called without creating the render target.

I tested on Windows 10 Pro 20H2. With this patch, all examples are successfully showing the window without panic.

[Ciantic]

This also fixes #1582. I'm okay with this patch for the time being (I don't know the side effects of creating DC during WM_CREATE either but it worked).

Reason is that way render_target is created is really unorthodox at the moment, I might make a bigger PR to refactor whole render_target creation. Usually DC or equivalent is created on demand during render (e.g. WM_PAINT).

Right now the render function just unwraps the render_target and crashes if it's not there. It should instead create the render target in render function if it doesn't exist.

[cmyr]

Okay, I'm happy to merge this as a hotfix with the understanding that we would like a more robust solution longer-term.