Ensure cinoptions is set to a known value (#24)

Fix `set cinoptions=#1` causes incorrect define indenting.

To get correct glsl indentation, we want default 'cinoptions'. It's useful to
`set cinoptions=#1` (to allow most filetypes to use # comments), so we should
ensure we're using a known set of indent options and revert to the default.

The user can still customize cinoptions in ~/.vim/after/indent/glsl.vim.

## Test
```vim
:set cinoptions=#1
:edit test.glsl
gg=G
```

```glsl
// test.glsl
#define PI 4
#define TAU 8 // line gets 1 space indent

float f(float x) {
	return x * 2;
}
```

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