vulkan: rename depth bias "constant" to "factor"

"constant" is a special keyword in OpenCL C, and we'd like to #define it suitably in host C23 to facilitate compatiblity between host/device headers. That means we can't have any identifiers named "global" or "constant". Fortunately, this is the only 'constant' in any file I'm hitting.

To avoid the clash, abbreviate "constant factor" as "factor" instead of "constant". That name is, I think, no worse.