Scanrate fix for visual debugger

[Waccoon]

• Initialize and clamp local maxhpos and maxvpos values if scanrate changes.
• Initialize "dma_rec->end" when recording DMA to prevent false hsync record if scanrate changes.

[Waccoon]

I noticed when changing the screenmode to DoubleNTSC and then back to NTSC, the maximum values were never clamped, resulting in the visual debugger filling the whole screen. I tested this fix with all different screenmodes including HD720 and HighGFX and it seems to work fine. As implemented, it's only important that all maximum dimensions are set correctly at the end of the frame on vsync, but the values still accumulate correctly per scanline.