[mutter] clutter: Align all screen transformations to 1/256th of a stage unit.

[verdre <jonasd src gnome org>]

commit 67cc60cbda29d3b0ad19aad6cdae88f25346f3b3
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Tue Sep 8 16:40:54 2020 +0800

    clutter: Align all screen transformations to 1/256th of a stage unit.
    
    So as to eliminate floating point precision errors that creep in
    during matrix operations.
    
    1/256th is chosen as a reasonable maximum resolution to cover any
    realistic fractional scaling factor. Floats can represent such
    a fraction losslessly because it is a small power of 2.
    
    Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1403
    
    https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1429

 clutter/clutter/clutter-util.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)
---
diff --git a/clutter/clutter/clutter-util.c b/clutter/clutter/clutter-util.c
index 6e59889a8c..d386b1db8e 100644
--- a/clutter/clutter/clutter-util.c
+++ b/clutter/clutter/clutter-util.c
@@ -47,6 +47,8 @@
 #define MTX_GL_SCALE_Y(y,w,v1,v2) ((v1) - (((((y) / (w)) + 1.0f) / 2.0f) * (v1)) + (v2))
 #define MTX_GL_SCALE_Z(z,w,v1,v2) (MTX_GL_SCALE_X ((z), (w), (v1), (v2)))
 
+#define ROUND_TO_256THS(x) (roundf ((x) * 256) / 256)
+
 void
 _clutter_util_fully_transform_vertices (const CoglMatrix *modelview,
                                         const CoglMatrix *projection,
@@ -99,10 +101,14 @@ _clutter_util_fully_transform_vertices (const CoglMatrix *modelview,
       ClutterVertex4 vertex_tmp = vertices_tmp[i];
       graphene_point3d_t *vertex_out = &vertices_out[i];
       /* Finally translate from OpenGL coords to window coords */
-      vertex_out->x = MTX_GL_SCALE_X (vertex_tmp.x, vertex_tmp.w,
-                                      viewport[2], viewport[0]);
-      vertex_out->y = MTX_GL_SCALE_Y (vertex_tmp.y, vertex_tmp.w,
-                                      viewport[3], viewport[1]);
+      vertex_out->x = ROUND_TO_256THS (MTX_GL_SCALE_X (vertex_tmp.x,
+                                                       vertex_tmp.w,
+                                                       viewport[2],
+                                                       viewport[0]));
+      vertex_out->y = ROUND_TO_256THS (MTX_GL_SCALE_Y (vertex_tmp.y,
+                                                       vertex_tmp.w,
+                                                       viewport[3],
+                                                       viewport[1]));
     }
 }