Fix linux build

[charles-lunarg]

Gets GLM on linux using FetchContent since the linux SDK doesn't contain it.

Makes non-negative VkResult return values not call exit, allowing resize to "work".

[charles-lunarg]

Wait, I might of fudged the non-negative vkresult value check...

[charles-lunarg]

NVM - I misread an outdated log.

[SaschaWillems]

Currently leaning towards installing all dependencies and mostly removing reliance on the SDK. Noticed that when e.g. behind restrictive firewalls the SDK is not able to fetch any of the non-core libraries like glm, volk and vma.

[mpalomas]

I have my own fork with a similar but better approach than this one.
I understand the motivation and firewall issue.
May I suggest:

• Either a small clarification in the README, saying the libs must be installed with the Vulkan SDK (EDIT: oh I just saw it's kind of done in the tutorial md)
• Or vendoring (maybe this is what you mean): since they are extremely small libs, just copy them into "external"

[charles-lunarg]

I've removed the change on how to check the return value of swapchain - your branch #16 does this better anyhow.