libicon: Add tool to encode/decode custom float format

I've encoded and decoded the HVIF format's custom three byte float
format by hand several times by now. This adds a python tool to
automatically encode and decode these.

Change-Id: I92cc1697a0b4fc3b55bb20a5bbcf6e93e3c0738b
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6448
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

src/tests/libs/icon/flat_icon/FlatIconFormat.py

@@ -0,0 +1,37 @@
+# Utility functions to aid in converting from the special data types the HVIF
+# (aka flat icon format) has
+
+import math
+
+# Example usage:
+# >>> from FlatIconFormat import *
+# >>> read_float_24(0x3ffffd)
+def read_float_24(value):
+	sign = (-1) ** ((value & 0x800000) >> 23)
+	exponent = 2 ** (((value & 0x7e0000) >> 17) - 32)
+	mantissa = (value & 0x01ffff) * 2 ** -17
+	return sign * exponent * (1 + mantissa)
+
+# Example usage:
+# >>> hex(write_float_24(0.9999885559082031))
+#
+# Does not perform bounds checking. Do not input numbers that are too large.
+def write_float_24(value):
+	# TODO: does not differentiate between 0.0 and -0.0
+	if value >= 0:
+		sign = 0
+	else:
+		sign = 1
+
+	if value != 0:
+		# TODO: make sure exponent fits in 6 bits
+		exponent = math.floor(math.log2(abs(value)))
+	else:
+		exponent = -32
+
+	if value != 0:
+		mantissa = abs(value) / 2**exponent - 1
+	else:
+		mantissa = 0
+
+	return (sign << 23) + ((exponent+32) << 17) + math.floor(mantissa * 2 ** 17)