GLSL: Add a option to fix up clip space.

OpenGL uses the NDC with z of [-1, 1], and Vulkan, DirectX, Metal use the NDC with z of [0, 1].

When we want fix a vertex shader originally written for OpenGL, we can add an extra line:

```glsl
gl_Position.z = (gl_Position.z + gl_Position.w) * 0.5;
```

Or generate SPIR-V code of this meaning.

[SPIRV-Cross has an option for it](https://github.com/KhronosGroup/SPIRV-Cross#clip-space-conventions).

Like #2936, I want use the same source code of shaders for OpenGL and Vulkan.