Fix minor build issue due to types mismatch.

Fixes the following build issue on OS X 10.10.1, Go 1.4.

```
gl32 $ go build
# github.com/GlenKelley/go-gl/gl32
./gl32.go:910: cannot use C.GLclampf(red) (type C.GLclampf) as type C.GLfloat in argument to _Cfunc_glClearColor
./gl32.go:910: cannot use C.GLclampf(green) (type C.GLclampf) as type C.GLfloat in argument to _Cfunc_glClearColor
./gl32.go:910: cannot use C.GLclampf(blue) (type C.GLclampf) as type C.GLfloat in argument to _Cfunc_glClearColor
./gl32.go:910: cannot use C.GLclampf(alpha) (type C.GLclampf) as type C.GLfloat in argument to _Cfunc_glClearColor
./gl32.go:1059: cannot use C.GLclampf(red) (type C.GLclampf) as type C.GLfloat in argument to _Cfunc_glBlendColor
./gl32.go:1059: cannot use C.GLclampf(green) (type C.GLclampf) as type C.GLfloat in argument to _Cfunc_glBlendColor
./gl32.go:1059: cannot use C.GLclampf(blue) (type C.GLclampf) as type C.GLfloat in argument to _Cfunc_glBlendColor
./gl32.go:1059: cannot use C.GLclampf(alpha) (type C.GLclampf) as type C.GLfloat in argument to _Cfunc_glBlendColor
```