Allow ScratchImage to be non-owning

[solbjorn]

ScratchImage is very convenient as it automatically compute the number of images, pitches etc. On the other hand, when the user has an already loaded texture, it still always allocate own buffer and perform copying, which might be suboptimal, especially with large textures.

Extend ScratchImage::Initialize() with an additional argument (with the default value that behaves the same way as before) to allow it to be non-owning. This means that it still computes the number of images, pitches etc., but doesn't allocate the actual buffer. Then, the user can update individual images using ScratchImage::SetNonOwningImagePixels().
The actual ScratchImage doesn't own this memory, doesn't have own buffer and doesn't perform any deallocation on destruction. Memory management and lifetimes are fully on the user's side.

A non-owning ScratchImage differs from owning by having a non-zero m_size and m_memory == nullptr.

An example of usage:

// create ScratchImage using Initialize(meta, cp_flags, false)

// set pixels pointer for each image using
auto& image = *scratch.SetNonOwningImagePixels(mip, face, depth, ptr);

The return value is optional and is the same as ScratchImage.GetImage(mip, face, depth), only to avoid 2 external calls per subimage when needed. For example, if the user has a contiguous buffer with a mipmapped 2D image:

for (size_t i{0}, off{0}; i < scratch.GetMetadata().mipLevels; ++i)
    off += scratch.SetNonOwningImagePixels(i, 0, 0, ptr + off)->slicePitch;

A non-owning ScratchImage can currently be initialized only using Initialize() for simplicity and to indicate that the option is for advanced users only. Other Initialize*() methods can be expanded later if needed.

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[solbjorn]

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