[opengl] Merge Args + Retr + Extr SSBO

[k-ye]

Here are the SSBOs currently in use:

taichi/taichi/backends/opengl/opengl_kernel_util.h

Lines 47 to 54 in f1fe2c6

	enum class GLBufId {
	Root = 0,
	Gtmp = 1,
	Args = 2,
	Retr = 3,
	Extr = 4,
	Runtime = 5,
	};

Args, Retr and Extr are all used to pass data between the kernel and the host. We can merge them to release some SSBO slots.

• Merge Args and Extr
• Merge Args and Retr