scons: Silence conversion from 'size_t' to 'type', possible loss of data on MSVC.

Most cases seem harmless, though that might not always be the case.  Maybe
one day we can get gcc to complain about these and fix them throughout
the code, but until then let's silence them.

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

scons/gallium.py

@@ -509,6 +509,7 @@ def generate(env):
             '/wd4018', # signed/unsigned mismatch
             '/wd4056', # overflow in floating-point constant arithmetic
             '/wd4244', # conversion from 'type1' to 'type2', possible loss of data
+            '/wd4267', # 'var' : conversion from 'size_t' to 'type', possible loss of data
             '/wd4305', # truncation from 'type1' to 'type2'
             '/wd4351', # new behavior: elements of array 'array' will be default initialized
             '/wd4756', # overflow in constant arithmetic