Compile with -Wfloat-conversion

This means we'll get warnings whenever a floating point value looses
precision, e.g. gets implicitly casted to an integer. It also warns when
implicitly casting double's to float's, which arguably is less of a
problem, but there are no warning for just float/double to int.

This would have caught
https://gitlab.gnome.org/GNOME/mutter/-/issues/3530.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3822>

src/backends/native/meta-kms-utils.c

@@ -36,7 +36,7 @@ meta_calculate_drm_mode_refresh_rate (const drmModeModeInfo *drm_mode)
   if (drm_mode->vscan > 1)
     denominator *= drm_mode->vscan;
 
-  return numerator / denominator;
+  return (float) (numerator / denominator);
 }
 
 int64_t