Output - Screenshot Imgui widget

[tiawl]

One of the spaceporn-cli main goal is to generate content. So because I am not totally comfortable with Vulkan API, I choosed to split this goal in 2:

• static content, only when all dynamic options are set to false.
• dynamic content, otherwise.

Static content will be the first main goal. Because PPM is an easy format to generate and an already covered topic here:

• https://github.com/SaschaWillems/Vulkan/blob/master/examples/renderheadless/renderheadless.cpp
• https://github.com/SaschaWillems/Vulkan/blob/master/examples/screenshot/screenshot.cpp

We are going for this format instead of PNG.

[tiawl]

Removing CLI option in favor of a dedicated Imgui widget