Looks for a uniform by the name name, and returns the index of the uniform, or -1 if it was not found.
GTK's new Vulkan-focused rendering does not support this feature. Use [GtkGLArea](../gtk4/class.GLArea.html) for OpenGL rendering.
| this |
a `GskGLShader` |
| name |
uniform name |
|
The index of the uniform, or -1 |