GPU: Rename GPU_shader_get_uniform to GPU_shader_get_uniform_ensure

This is in order to make the API more multithread friendly inside the
draw manager.

GPU_shader_get_uniform will only serve to query the shader interface and
not do any GL call, making it threadsafe.

For now it only print a warning if the uniform was not queried before.

source/blender/python/gpu/gpu_py_shader.c

@@ -86,7 +86,7 @@ success:
 
 static int bpygpu_uniform_location_get(GPUShader *shader, const char *name, const char *error_prefix)
 {
-	int uniform = GPU_shader_get_uniform(shader, name);
+	int uniform = GPU_shader_get_uniform_ensure(shader, name);
 
 	if (uniform == -1) {
 		PyErr_Format(PyExc_ValueError, "%s: uniform %.32s %.32s not found", error_prefix, name);