allow more granular background orientations

Since bitmaps, suns, and skyboxes store their coordinates as floating-point numbers, we can allow mission designers to specify floating-point numbers in the FRED UI.  This PR modifies the dialogs in FRED and QtFRED to allow two decimal places for pitch, bank, and heading.

Implements #7170.

Author: Goober5000

code/math/floating.h

@@ -16,6 +16,8 @@
 #include <cfloat>
 #include <limits>
 
+#include "globalincs/pstypes.h"		// for PI
+
 extern float frand();
 extern int rand_chance(float frametime, float chance = 1.0f);
 float frand_range(float min, float max);
@@ -51,6 +53,20 @@ inline bool fl_is_nan(float fl) {
 // convert a measurement in radians to degrees
 #define fl_degrees(fl)	(static_cast<float>((fl) * (180.0f / PI)))
 
+constexpr float DEGREE_UB = 359.99f;	// upper bound in degrees when rounding to 100ths place
+
+// convert a measurement in radians to degrees, rounding to the nearest .01 (used in FRED)
+constexpr float fl_degrees_100ths(float fl)
+{
+	float val = fl_degrees(fl);
+
+	float new_val = fl2ir(val * 100.0f) / 100.0f;
+	if (new_val > DEGREE_UB)
+		new_val -= 360.0f;
+
+	return new_val;
+}
+
 // sees if a floating point number is within a certain threshold (by default, epsilon) of zero
 inline bool fl_near_zero(float a, float e = std::numeric_limits<float>::epsilon())
 {