Fix double-promotion warnings

Make it an error.

src/hb-ot-var-fvar-table.hh

diff --git a/src/hb-ot-var-fvar-table.hh b/src/hb-ot-var-fvar-table.hh
index 3565185..c73348e 100644
--- a/src/hb-ot-var-fvar-table.hh
+++ b/src/hb-ot-var-fvar-table.hh
@@ -120,10 +120,10 @@
     const AxisRecord &axis = get_axes ()[axis_index];
     info->tag = axis.axisTag;
     info->name_id =  axis.axisNameID;
-    info->default_value = axis.defaultValue / 65536.;
+    info->default_value = axis.defaultValue / 65536.f;
     /* Ensure order, to simplify client math. */
-    info->min_value = hb_min (info->default_value, axis.minValue / 65536.);
-    info->max_value = hb_max (info->default_value, axis.maxValue / 65536.);
+    info->min_value = hb_min (info->default_value, axis.minValue / 65536.f);
+    info->max_value = hb_max (info->default_value, axis.maxValue / 65536.f);
   }
 
   void get_axis_info (unsigned int axis_index,
@@ -134,10 +134,10 @@
     info->tag = axis.axisTag;
     info->name_id =  axis.axisNameID;
     info->flags = (hb_ot_var_axis_flags_t) (unsigned int) axis.flags;
-    info->default_value = axis.defaultValue / 65536.;
+    info->default_value = axis.defaultValue / 65536.f;
     /* Ensure order, to simplify client math. */
-    info->min_value = hb_min (info->default_value, axis.minValue / 65536.);
-    info->max_value = hb_max (info->default_value, axis.maxValue / 65536.);
+    info->min_value = hb_min (info->default_value, axis.minValue / 65536.f);
+    info->max_value = hb_max (info->default_value, axis.maxValue / 65536.f);
     info->reserved = 0;
   }
 

src/hb.hh

diff --git a/src/hb.hh b/src/hb.hh
index 96db1f9..3d4b180 100644
--- a/src/hb.hh
+++ b/src/hb.hh
@@ -65,6 +65,7 @@
 #pragma GCC diagnostic error   "-Wcast-align"
 #pragma GCC diagnostic error   "-Wcast-function-type"
 #pragma GCC diagnostic error   "-Wdelete-non-virtual-dtor"
+#pragma GCC diagnostic error   "-Wdouble-promotion"
 #pragma GCC diagnostic error   "-Wformat-security"
 #pragma GCC diagnostic error   "-Wimplicit-function-declaration"
 #pragma GCC diagnostic error   "-Winit-self"