mesa: Fix _mesa_float_to_unorm() on 32-bit systems.

This fixes the following CTS test on 32-bit systems:
GTF-GL46.gtf30.GL3Tests.packed_depth_stencil.packed_depth_stencil_init

It does glGetTexImage of a 16-bit SNORM image, requesting 32-bit UNORM
data.  In get_tex_rgba_uncompressed, we round trip through float to
handle image transfer ops for clamping.  _mesa_format_convert does:

   _mesa_float_to_unorm(0.571428597f, 32)

which translated to:

   _mesa_lroundevenf(0.571428597f * 0xffffffffu)

which produced different results on 64-bit and 32-bit systems:

   64-bit: result = 0x92492500
   32-bit: result = 0x80000000

This is because the size of "long" varies between the two systems, and
0x92492500 is too large to fit in a signed 32-bit integer.  To fix this,
we switch to the new _mesa_i64roundevenf function which always does the
64-bit operation.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104395


Fixes: 594fc0f8 ("mesa: Replace F_TO_I() with _mesa_lroundevenf().")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit e18cd545)