[glsl-in] Expression ownership

[pjoe]

Just copy pasted info for easy ref:

GLSL-in shoud roughly following wgsl-in:

1. Generate Expression::Load whenever we operate on a value of a variable (both locals and globals)
2. Track the expressions we produce (possibly using Emitter struct that's currently shared between spv-in and wgsl-in):

• flush (into Statement::Emit) before adding a statement using any of the expressions
• exclude the Expression::Constant and Expression::Call by flushing before we add them, and restarting the emitter afterwards

[monocodus]

This is an autogenerated code review.

Checker summary (by rust_clippy):
The tool has found 3 warnings, 0 errors.

[JCapucho]

Closing this since #889 already implements this for the new parser