Returns the number of textures that the shader requires.
GTK's new Vulkan-focused rendering does not support this feature. Use [GtkGLArea](../gtk4/class.GLArea.html) for OpenGL rendering.
This can be used to check that the a passed shader works in your usecase. It is determined by looking at the highest u_textureN value that the shader defines.
| this |
a `GskGLShader` |
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The number of texture inputs required by this |