Pre-rotate MyPaint tilt inputs

To get sensible tilt results when rotating the canvas. Since we already
have to mirror and flip them this way in absence of any other way to
input that information into libmypaint, will probably give consistent
results. It's not what MyPaint does, but it also doesn't give proper
results when rotating the canvas.

Relates to #1423.

src/drawdance/libengine/dpengine/brush_engine.c

@@ -1365,15 +1365,46 @@ static void stroke_to_classic(
 }
 
 
+static void tilt_to_mypaint(float xtilt, float ytilt, float angle, bool mirror,
+                            bool flip, float *out_xtilt, float *out_ytilt)
+{
+    if (xtilt == 0.0f && ytilt == 0.0f) {
+        *out_xtilt = 0.0f;
+        *out_ytilt = 0.0f;
+    }
+    else {
+        float x, y;
+        if (angle == 0.0f) {
+            x = xtilt;
+            y = ytilt;
+        }
+        else {
+            float orientation = atan2f(ytilt, xtilt);
+            float magnitude = hypotf(xtilt, ytilt);
+            if (mirror == flip) {
+                orientation -= angle;
+            }
+            else {
+                orientation += angle;
+            }
+            x = cosf(orientation) * magnitude;
+            y = sinf(orientation) * magnitude;
+        }
+        *out_xtilt = CLAMP(x, -60.0f, 60.0f) / (mirror ? -60.0f : 60.0f);
+        *out_ytilt = CLAMP(y, -60.0f, 60.0f) / (flip ? -60.0f : 60.0f);
+    }
+}
+
 static void stroke_to_mypaint(DP_BrushEngine *be, DP_BrushPoint bp)
 {
     MyPaintBrush *mb = be->mypaint_brush;
     MyPaintSurface2 *surface = &be->mypaint_surface2;
     float zoom = be->stroke.zoom;
     float angle = be->stroke.angle_rad;
-    float xtilt = bp.xtilt / (be->stroke.mirror ? -60.0f : 60.0f);
-    float ytilt = bp.ytilt / (be->stroke.flip ? -60.0f : 60.0f);
     float rotation = bp.rotation / 360.0f;
+    float xtilt, ytilt;
+    tilt_to_mypaint(bp.xtilt, bp.ytilt, angle, be->stroke.mirror,
+                    be->stroke.flip, &xtilt, &ytilt);
 
     double delta_sec;
     if (be->stroke.in_progress) {