mesa: Silence narrowing warnings in ff_fragment_shader's emit_texenv().

mesa: Silence narrowing warnings in ff_fragment_shader's emit_texenv().

Recent version of GCC report a warning for the implicit conversion from
int to float:

  ff_fragment_shader.cpp:897:3: warning: narrowing conversion of '(1 << ((int)rgb_shift))' from 'int' to 'float' inside { } is ill-formed in C++11 [-Wnarrowing]

This is because floats cannot precisely represent all possible 32-bit
integer values.  However, texenv code is all expected to be floating
point, so this should not be a problem.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>