minor edits / cleanup - no functional changes.

- use 'const float *' and array size in some function declarations.
- replace macros for BLI_math functions INPF, VECCOPY, VECADD etc.
- remove unused VertRen.clip struct member.
- remove static squared_dist() from 2 files, replace with BLI_math function len_squared_v3v3().
- use vertex arrays for drawing clipping background in the 3D viewport.

source/blender/editors/space_node/drawnode.c

@@ -305,7 +305,7 @@ static void node_buts_curvecol(uiLayout *layout, bContext *UNUSED(C), PointerRNA
 
 	if(_sample_col) {
 		cumap->flag |= CUMA_DRAW_SAMPLE;
-		VECCOPY(cumap->sample, _sample_col);
+		copy_v3_v3(cumap->sample, _sample_col);
 	}
 	else 
 		cumap->flag &= ~CUMA_DRAW_SAMPLE;