scons: Adjust the warnings for VS.

Silence insignificant warnings so significant warnings have a chance to
stand out.

The only abundant warning that's not silenced here is "C4018:
signed/unsigned mismatch", as it could hide security issues, so it's better
to actually fix the code.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>

scons/gallium.py

@@ -455,7 +455,9 @@ def generate(env):
             ]
         ccflags += [
             '/W3', # warning level
-            #'/Wp64', # enable 64 bit porting warnings
+            '/wd4244', # conversion from 'type1' to 'type2', possible loss of data
+            '/wd4305', # truncation from 'type1' to 'type2'
+            '/wd4800', # forcing value to bool 'true' or 'false' (performance warning)
             '/wd4996', # disable deprecated POSIX name warnings
         ]
         if env['machine'] == 'x86':