New tests for location overlap with scalars, doubles and for
duplicated inputs which match the same output variable in the previous
stage.

From the ARB_separate_shader_objects spec v.25:

  "   * An output variable is considered to match an input variable in
        the subequent shader if:

        * the two variables match in name, type, and qualification; or

        * the two variables are declared with the same location layout
          qualifier and match in type and qualification."

...

  " A program will fail to link if any two non-vertex shader input
    variables are assigned to the same location."

Cc: Timothy Arceri <tarceri@itsqueeze.com>
Cc: Iago Toral Quiroga <itoral@igalia.com>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>