Fix issue #1574 "fract in linalg/glm is broken" by fixing

trunc_f16/32/64 in "math.odin" (~ typos on expressions)
Fix classify_f16 Inf test (would fail for subnormal 0h0001)
  by changing multiplier 0.5 -> 0.25
Add some useful consts to "math.odin" (INF_F16 etc)
Add comment to "demo.odin" mentioning that -0.0 must be used
  to specify negative zero

examples/demo/demo.odin

@@ -94,6 +94,7 @@ the_basics :: proc() {
 		z: f64 // `z` is typed of type `f64` (64-bit floating point number)
 		z = 1  // `1` is an untyped integer literal which can be implicitly converted to `f64`
 				// No need for any suffixes or decimal places like in other languages
+				// (with the exception of negative zero, which must be given as `-0.0`)
 				// CONSTANTS JUST WORK!!!