`flip_vert_y` for geometry shaders does not produce expected results

The current `vertex.flip_vert_y` implementation adds `gl_Position.y` flipping at `SPIRBlock::Return`. https://github.com/KhronosGroup/SPIRV-Cross/blob/72b5b7c1a604592d98e7d4b0883231274122fd29/spirv_glsl.cpp#L17911-L17917

However, this does not affect the `EmitVertex` calls, so the vertices emitted in geometry shaders remain unflipped, which appears to be unintended.

Relevant issue: https://github.com/KhronosGroup/SPIRV-Cross/issues/1469

Small thoughts:
- Basically we could flip Y just before every `EmitVertex` call to resolve this issue. However, the code may use `gl_Position` after the `EmitVertex` call (I won't write such a code though), so the modification should be rolled back immediately to maintain the original behavior.
- For VS-GS or VS-TCS-TES (or similar) pipelines, we can refer to the `gl_Position` output from the previous stage. So it might be necessary to flip the input initially. However, if the geometry shader is used in isolation, flipping the input is not necessary...
- It might be useful to add an option to specify how the input should be handled...

Start in spirv_glsl.cpp at the linked SPIRBlock::Return code and trace how EmitVertex calls are generated. Determine how geometry-shader vertex emission interacts with the existing gl_Position flip, including later uses of gl_Position and the input-stage cases described in the issue; done means the expected Y orientation is preserved without changing other observed behavior.

Written by the indexing model from the issue text.