Add struct scope variable masking test.

GLSL allows the shader to define a new inner struct with the same name
as an outer struct. Moreover, you can declare variables of the outer
type, then redeclare the struct, then declare a variable with the new
type. Currently this is broken in ANGLE.
Merge pull request #575 from jamiemadill/scoped-struct-masking

Add struct scope variable masking test.