Fixing retrospective code for specialisation constants

[BastiaanOlij]

This was a weird on but it caused the vulkan mobile renderer to fail rendering lights for ages.

When parsing the compiled shader to obtain the default values for our specialization constants, when assigning a bool it failed if the previous values was a floating point value. It may also fail with ints but I didn't test all edge cases.

My theory here is that because any non-zero value is true, setting the bool was getting confused. The fix was really simple, just reset it's current value before the boolean is assigned.

I also added SpvReflectSpecializationConstant *spc = spec_constants[j]; because it made debugging easier though debugging this is a pain because this code is threaded.
The real fix is just the onliner sconst.int_value = 0.0; // clear previous value JIC before the case statement :)

[akien-mga]

Thanks!