fix #210 when starting and resizing, waterfall appears garbled

[cybork42]

It seems there was some migration issue left behind when implementing device context handling with wxWidgets.
The waterfall is an area of its own and odes not have any borders or such.
The old code was
gc->DrawRectangle(PLOT_BORDER + XLEFT_OFFSET, PLOT_BORDER + YBOTTOM_OFFSET, m_imgWidth, m_imgHeight);
The correct one seems to be:
gc->DrawRectangle(0, 0, m_imgWidth, m_imgHeight); // DC waterfall bitmap starts at origin and spans full width

[drowe67]

Thanks @cybork42. Unfortunately I can't reproduce #210 on my machine using master so I can't determine if this PR fixes the issue. @tmiw can you reproduce the issue?

[tmiw]

Thanks @cybork42. Unfortunately I can't reproduce #210 on my machine using master so I can't determine if this PR fixes the issue. @tmiw can you reproduce the issue?

I could never reproduce this issue myself but the changes in this PR seem okay on macOS and Ubuntu at least.

Also, looking at the code, it does make sense to draw on the bitmap at 0,0. Note that the bitmap is then drawn in the correct location further down:

        // Reset bitmap to black.   
        {
            wxMemoryDC dc(*m_fullBmp);
            wxGraphicsContext *mGc = wxGraphicsContext::Create( dc );
        
            wxBrush ltGraphBkgBrush = wxBrush(BLACK_COLOR);
            mGc->SetBrush(ltGraphBkgBrush);
            mGc->SetPen(wxPen(BLACK_COLOR, 0));
            mGc->DrawRectangle(0, 0, m_imgWidth, m_imgHeight); // DC waterfall bitmap starts at origin and spans full width
            delete mGc;
        }
    }

    ...

    wxGraphicsBitmap tmpBmp = gc->CreateBitmap(*m_fullBmp);
    gc->DrawBitmap(tmpBmp, PLOT_BORDER + XLEFT_OFFSET, PLOT_BORDER + YBOTTOM_OFFSET, m_imgWidth, m_imgHeight);
    m_dT = DT;