Vulkan: fix issue with MSAA sidecar dimensions.
Vulkan: fix issue with MSAA sidecar dimensions.

Every sidecar texture should have the same dimensions as its parent
texture.

Prior to this change, the sidecar texture would have the dimensions of
the RenderTarget wrapper rather than the parent texture. This was wrong
because the RenderTarget might be slightly smaller than the texture.
This caused an issue when the same texture was attached to two render
targets with different sizes, because there's only 1 cached sidecar per
texture.

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