Run with reduced frame rate in warp mode

[dirkwhoffmann]

VirtualC64 has the option to run the VICII backend in warp mode with a lower frequency:

It should be easy to implement something similar for Denise. Replacing

    // Check if we are below the VBLANK area
    if (vpos >= 26) {

with something like the following should do the trick:

    // Check if we are below the VBLANK area
    if (vpos >= 26 && (!amiga.isWarping() || agnus.pos.frame % 16 == 0)) {

A minor issue remains. If a frame is skipped, swapping the working buffer with the stable buffer must be disabled (i.e., PixelEngine::swapBuffers() must not be executed). Otherwise, the display may flicker because swapping the buffer would replace a newer texture with an older one every other frame.

[dirkwhoffmann]

As expected, frame skipping results in significant performance improvements:

Reference (vAmiga 2.5): approx. 85 MHz in warp mode

Patched version with frame skipping (pixel engine stuff is only run for every 16th frame): approx 99 MHz