Previously we were using layout.layer_size for the layer stride, but
in Vulkan, you can alias a 3D image as an array of 2D images via the
VK_IMAGE_CREATE_2D_ARRAY_COMPATIBLE_BIT flag. One reason to use
this behavior is so the geometry shader can write to a specific
depth in a 3D framebuffer with gl_Layer.
Since the 3D image is not a *true* layered image, layer_size is 0.
Instead, we can copy what freedreno does and use the slice size.
Fixes dEQP-VK.geometry.layered.3d.*
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4541>
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