[glsl-in] Implement swizzle for r-values (#213)

* [glsl-in] Implement swizzle for r-values

Related to #210

* [glsl-in] Just return Result from field_selection

Removed unneccessary Otion in return type

* [glsl-in] Always match on type in field_selection

* [glsl.in] Borrow by value in field_selection