util: add fp64 -> fp32 conversion support for RTNE and RTZ rounding modes

In order to be coherent with the pre-existent API for half floats,
this new API for double is the one meant to be used when doing double
to float conversions. It is no more than a wrapper for the softfloat.h
API but we meant to keep that one private.

v2:
- Fix bug in _mesa_double_to_float_rtz() in the inf/nan detection
  using the exponent value.

v3:
- Replace custom f64 -> f32 implementations with the softfloat
  one (Andres).

v4:
- Added API usage clarifying comments (Caio).

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>

src/util/double.c

@@ -0,0 +1,38 @@
+/*
+ * Mesa 3-D graphics library
+ *
+ * Copyright (C) 2018-2019 Intel Corporation
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included
+ * in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+ * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
+ * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+ * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ * OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#include "softfloat.h"
+#include "double.h"
+
+float
+_mesa_double_to_float(double val)
+{
+    return _mesa_double_to_f32(val, false);
+}
+
+float
+_mesa_double_to_float_rtz(double val)
+{
+    return _mesa_double_to_f32(val, true);
+}