Enable zero padding with float specifier for format strings

Godot currently supports zero padding for integers, octals and
hexadecimals when using format strings, but not for floats.

This commit adds support for zero padding for floats, thus ensuring
consistent behavior for all types, and making Godot's format specifiers'
behavior closer to c's `printf()`.

Before: `print("<%07.2f>" % -0.2345)` prints `<  -0.23>`.
Now: `print("<%07.2f>" % -0.2345)` prints `<-000.23>`.
     `print("<%7.2f>" % -0.2345)` prints `<  -0.23>`.
(cherry picked from commit b7d835d9cae8625fe32fa837bae6f3e7843f50e7)

core/ustring.cpp

@@ -4211,27 +4211,40 @@ String String::sprintf(const Array &values, bool *error) const {
 					}
 
 					double value = values[value_index];
-					String str = String::num(value, min_decimals);
+					bool is_negative = (value < 0);
+					String str = String::num(abs(value), min_decimals);
 
 					// Pad decimals out.
 					str = str.pad_decimals(min_decimals);
 
-					// Show sign
-					if (show_sign && str.left(1) != "-") {
-						str = str.insert(0, "+");
-					}
+					int initial_len = str.length();
 
-					// Padding
+					// Padding. Leave room for sign later if required.
+					int pad_chars_count = (is_negative || show_sign) ? min_chars - 1 : min_chars;
+					String pad_char = pad_with_zeroes ? String("0") : String(" ");
 					if (left_justified) {
-						str = str.rpad(min_chars);
+						if (pad_with_zeroes) {
+							return "left justification cannot be used with zeros as the padding";
+						} else {
+							str = str.rpad(pad_chars_count, pad_char);
+						}
 					} else {
-						str = str.lpad(min_chars);
+						str = str.lpad(pad_chars_count, pad_char);
+					}
+
+					// Add sign if needed.
+					if (show_sign || is_negative) {
+						String sign_char = is_negative ? "-" : "+";
+						if (left_justified) {
+							str = str.insert(0, sign_char);
+						} else {
+							str = str.insert(pad_with_zeroes ? 0 : str.length() - initial_len, sign_char);
+						}
 					}
 
 					formatted += str;
 					++value_index;
 					in_format = false;
-
 					break;
 				}
 				case 's': { // String