[instancer-solver] port solver fix from fonttools

See fonttools/fonttools#3139

src/hb-subset-instancer-solver.cc

@@ -173,7 +173,7 @@ _solve (Triple tent, Triple axisLimit, bool negative = false)
     // Crossing point on the axis.
     float crossing = peak + ((1 - gain) * (upper - peak) / (1 - outGain));
 
-    Triple loc{peak, peak, crossing};
+    Triple loc{axisDef, peak, crossing};
     float scalar = 1.f;
 
     // The part before the crossing point.

src/test-subset-instancer-solver.cc

@@ -331,6 +331,24 @@ main (int argc, char **argv)
     assert (out[0].second == Triple (0.f, 0.4f, 32767/(float) (1 << 14)));
   }
 
+
+  {
+    Triple tent (0.f, 0.5f, 1.f);
+    Triple axis_range (-1.f, 0.25f, 1.f);
+    result_t out = rebase_tent (tent, axis_range);
+    assert (out.length == 5);
+    assert (out[0].first == 0.5f);
+    assert (out[0].second == Triple ());
+    assert (out[1].first == 0.5f);
+    assert (out[1].second == Triple (0.f, 1.f/3, 2.f/3));
+    assert (out[2].first == -0.5f);
+    assert (out[2].second == Triple (2.f/3, 1.f, 1.f));
+    assert (out[3].first == -0.5f);
+    assert (out[3].second == Triple (-1.f, -0.2f, 0.f));
+    assert (out[4].first == -0.5f);
+    assert (out[4].second == Triple (-1.f, -1.f, -0.2f));
+  }
+
   {
     Triple tent (0.5f, 0.5f, 0.5f);
     Triple axis_range (0.f, 0.5f, 1.f);