Metal: vertex descriptor is built without using the location information

[zeux]

This is the code that creates Metal vertex descriptors:

gfx/src/backend/metal/src/device.rs

Lines 1011 to 1014 in a571ae5

	let mtl_attribute_desc = vertex_descriptor
	.attributes()
	.object_at(i)
	.expect("too many vertex attributes");

The expected behavior is that the location in the vertex shader matches the location in the attributes array; instead this code fills the array from 0 to count-1, which doesn't work for vertex shaders that have 3 input attributes with locations 0, 2, 4. The array index should instead use VkVertexInputAttributeDescription::location.

[kvark]

@zeux this is an unbelievably precise definition of a bug, almost carrying a fix in it self :). Thank you for chasing this down! Fix is on the way.

[zeux]

Thanks! FWIW based on cursory inspection CTS doesn’t seem to cover this, I filed KhronosGroup/VK-GL-CTS#121 for this as well.