This change introduces program-type aware shader code replacement (#2424)

in VShaderWrapper, allowing independent GLSL modifications for:

- stroke programs
- fill programs
- depth programs

Previously, `set_color_by_code()` relied on a single global
replacement mechanism. As a result:

- Multiple calls to `set_color_by_code()` would override each other.
- Applying different GLSL logic to stroke and fill was not possible.
- The last shader replacement always won.

This update solves that limitation by:

1. Storing shader code replacements per program type.
2. Applying replacements only to shaders matching the given program_type.
3. Preserving previously applied replacements for other program types.
4. Maintaining backward compatibility when program_type is None.

Now users can safely do:

    set_color_by_code(..., program_type="stroke")
    set_color_by_code(..., program_type="fill")

without one affecting the other.

This makes stroke and fill shader logic fully independent,
opening the door for:

- Different gradients for stroke and fill
- Custom sheen effects per layer
- Advanced shader experimentation
- Cleaner shader architecture

Backward compatibility:
If no program_type is provided, behavior falls back to the
original global replacement logic.

This improves flexibility without breaking existing scenes.