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

tests/core/math/linalg/glsl/test_linalg_glsl_math.odin

@@ -0,0 +1,85 @@
+// Tests "linalg_glsl_math.odin" in "core:math/linalg/glsl".
+// Must be run with `-collection:tests=` flag, e.g.
+// ./odin run tests/core/math/linalg/glsl/test_linalg_glsl_math.odin -collection:tests=./tests
+package test_core_math_linalg_glsl_math
+
+import glsl "core:math/linalg/glsl"
+
+import "core:fmt"
+import "core:math"
+import "core:testing"
+import tc "tests:common"
+
+main :: proc() {
+
+    t := testing.T{}
+
+	test_fract_f32(&t)
+	test_fract_f64(&t)
+
+	tc.report(&t)
+}
+
+@test
+test_fract_f32 :: proc(t: ^testing.T) {
+
+	using math
+
+	r: f32
+
+	Datum :: struct {
+		i: int,
+		v: f32,
+		e: f32,
+	}
+	@static data := []Datum{
+		{ 0, 10.5, 0.5 }, // Issue #1574 fract in linalg/glm is broken
+		{ 1, -10.5, -0.5 },
+		{ 2, F32_MIN, F32_MIN }, // 0x1p-126
+		{ 3, -F32_MIN, -F32_MIN },
+		{ 4, 0.0, 0.0 },
+		{ 5, -0.0, -0.0 },
+		{ 6, 1, 0.0 },
+		{ 7, -1, -0.0 },
+		{ 8, 0h3F80_0001, 0h3400_0000 }, // 0x1.000002p+0, 0x1p-23
+		{ 9, -0h3F80_0001, -0h3400_0000 },
+	}
+
+	for d, i in data {
+		assert(i == d.i)
+		r = glsl.fract(d.v)
+		tc.expect(t, r == d.e, fmt.tprintf("i:%d %s(%v (%h)) -> %v (%h) != %v", i, #procedure, d.v, d.v, r, r, d.e))
+	}
+}
+
+@test
+test_fract_f64 :: proc(t: ^testing.T) {
+
+	using math
+
+	r: f64
+
+	Datum :: struct {
+		i: int,
+		v: f64,
+		e: f64,
+	}
+	@static data := []Datum{
+		{ 0, 10.5, 0.5 }, // Issue #1574 fract in linalg/glm is broken
+		{ 1, -10.5, -0.5 },
+		{ 2, F64_MIN, F64_MIN }, // 0x1p-1022
+		{ 3, -F64_MIN, -F64_MIN },
+		{ 4, 0.0, 0.0 },
+		{ 5, -0.0, -0.0 },
+		{ 6, 1, 0.0 },
+		{ 7, -1, -0.0 },
+		{ 8, 0h3FF0_0000_0000_0001, 0h3CB0_0000_0000_0000 }, // 0x1.0000000000001p+0, 0x1p-52
+		{ 9, -0h3FF0_0000_0000_0001, -0h3CB0_0000_0000_0000 },
+	}
+
+	for d, i in data {
+		assert(i == d.i)
+		r = glsl.fract(d.v)
+		tc.expect(t, r == d.e, fmt.tprintf("i:%d %s(%v (%h)) -> %v (%h) != %v", i, #procedure, d.v, d.v, r, r, d.e))
+	}
+}