Adds support for making custom memory allocations and images

[ewoudje]

The use I'm using it for is to have external images using a Win32 handle. Because this is very platform specific and niche I instead made a way to import self-made images and memory.

Specifically i exposed the vulkan device, the Allocation struct, allocate_memory and free_memory.
I added 2 functions to the vulkan context for importing memory and textures.
And added select_memory_type as a utility function.

I also added a way to add extensions when creating the context using a new struct PlatformSpecificContextDesc.
Which is used by the new function init_with_platform_desc. The original init function is still the same so it doesn't break anything.