Fix compiler warnings in testvulkan.c MSVC gives the following warnings when compiling testvulkan.c: ``` testvulkan.c(656,1): warning C4018: '<': signed/unsigned mismatch testvulkan.c(656,1): warning C4018: '>': signed/unsigned mismatch testvulkan.c(660,1): warning C4018: '<': signed/unsigned mismatch testvulkan.c(660,1): warning C4018: '>': signed/unsigned mismatch ```