PyGPU: call 'GPU_shader_bind' in 'GPUShader.uniform_' methods

This simplifies python code.

When we call a method like shader.uniform_float("color", (1,1,1,1)),
we expect the shader's uniform to be updated regardless of whether the
shader is bound or not.

And `batch.draw()` already calls `GPU_shader_bind` inside.

Differential Revision: https://developer.blender.org/D15929

doc/python_api/examples/gpu.6.py

@@ -56,7 +56,6 @@ batch = batch_for_shader(
 
 
 def draw():
-    shader.bind()
     shader.uniform_sampler("image", texture)
     batch.draw(shader)