[client] wayland: improve fractional scaling

avoids the 24.8 quantisation without introducing floating-point
equality/rounding behaviour throughout the Wayland state.

Closes #1257

client/displayservers/Wayland/scale.h

@@ -0,0 +1,80 @@
+/**
+ * Looking Glass
+ * Copyright © 2017-2026 The Looking Glass Authors
+ * https://looking-glass.io
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc., 59
+ * Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#include <stdint.h>
+
+struct WaylandScale
+{
+  int32_t num;
+  int32_t den;
+};
+
+static inline struct WaylandScale waylandScaleFromInt(int32_t scale)
+{
+  return (struct WaylandScale) { scale, 1 };
+}
+
+static inline struct WaylandScale waylandScaleFromRatio(int32_t num, int32_t den)
+{
+  return (struct WaylandScale) { num, den };
+}
+
+static inline bool waylandScaleValid(struct WaylandScale scale)
+{
+  return scale.num > 0 && scale.den > 0;
+}
+
+static inline bool waylandScaleEqual(struct WaylandScale a, struct WaylandScale b)
+{
+  return (int64_t)a.num * b.den == (int64_t)b.num * a.den;
+}
+
+static inline int waylandScaleCmp(struct WaylandScale a, struct WaylandScale b)
+{
+  int64_t lhs = (int64_t)a.num * b.den;
+  int64_t rhs = (int64_t)b.num * a.den;
+
+  return (lhs > rhs) - (lhs < rhs);
+}
+
+static inline bool waylandScaleIsFractional(struct WaylandScale scale)
+{
+  return waylandScaleValid(scale) && scale.num % scale.den != 0;
+}
+
+static inline int waylandScaleFloor(struct WaylandScale scale)
+{
+  return scale.num / scale.den;
+}
+
+static inline int waylandScaleCeil(struct WaylandScale scale)
+{
+  return (scale.num + scale.den - 1) / scale.den;
+}
+
+static inline int waylandScaleMulInt(struct WaylandScale scale, int value)
+{
+  return (int)(((int64_t)value * scale.num) / scale.den);
+}
+
+static inline double waylandScaleToDouble(struct WaylandScale scale)
+{
+  return (double)scale.num / (double)scale.den;
+}