mesa: use fi_type in vertex attribute code

mesa: use fi_type in vertex attribute code

For 32-bit builds, floating point operations use x86 FPU registers,
not SSE registers.  If we're actually storing an integer in a float
variable, the value might get modified when written to memory.  This
patch changes the VBO code to use the fi_type (float/int union) to
store/copy vertex attributes.

Also, this can improve performance on x86 because moving floats with
integer registers instead of FP registers is faster.

Neil Roberts review:
- include changes on all places that are storing attribute values.
- check with and without -O3 compiler flag.
Brian Paul review:
- use fi_type type instead gl_constant_value type
- fix a bunch of nit-picks.
- fix compiler warnings

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82668
Signed-off-by: Marius Predut <marius.predut@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>