Gets the value of the uniform idx in the args block.
GTK's new Vulkan-focused rendering does not support this feature. Use [GtkGLArea](../gtk4/class.GLArea.html) for OpenGL rendering.
The uniform must be of float type.
| this |
a `GskGLShader` |
| args |
uniform arguments |
| idx |
index of the uniform |
|
The value |