Fix hangs and segfaults when the FFT is moved around and zoomed

Moving the FFT plot off the screen caused a hang or a segfault. Making
the window too large caused segfaults because MAX_SCREENSIZE and
HORZ_DIVS_MAX were not taken into account. The zoom could also become
negative or get stuck on max.
Limit the max zoom-out to fix a crash

If the fft plot is zoomed out too far then there is an integer
overflow when computing m_pTranslateTbl, which results in memory
corruption when writing to outBuf.
Place limits on the fft plot dB range zoom

The fft plot dB range would get stuck if zoomed in or out too far. Put
limits on the range to prevent this from happening.
Merge pull request #65 from weinholt/master

Various fixes for a UI problems.