Draw: Removal of MultiSample Buffers

Multisample buffers were used for smooth line drawing. As we now have
an algorithm that doesn't need the multisample buffers we can remove
them.

The user preference for viewport multi_sampling is replaced by single
toggle overlay `use_overlay_smooth_wire`. By default this setting is
enabled as the new drawing is really quick (<1ms) and uses zero hacks.

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D6367

source/blender/blenkernel/BKE_sequencer.h

@@ -531,7 +531,6 @@ typedef struct ImBuf *(*SequencerDrawView)(struct Depsgraph *depsgraph,
                                            unsigned int flag,
                                            unsigned int draw_flags,
                                            int alpha_mode,
-                                           int samples,
                                            const char *viewname,
                                            struct GPUOffScreen *ofs,
                                            char err_out[256]);