- raster/BMP/GIF/PCX/TIF: fix dealing with very large data (use

`size_t` as appropriate)
- BMP: lessen heap memory usage by only `malloc()`ing a row, not
  whole file
- GIF: lessen heap memory usage by paging (also simplifies some
  function returns); use standard colour char map
- raster: add `raster_malloc()` to fail > 1GB (avoids very large
  output files that most systems can't handle; also lessens to
  some degree chances of being victim of OOM killer on Linux)
- GUI: printing scale dialog: set maxima on X-dim and resolution
  to keep scale <= 200

frontend_qt/scalewindow.h

@@ -57,6 +57,7 @@ private:
     float get_x_dim_mm() const;
     float get_dpmm() const;
     const char *getFileType() const;
+    void set_maxima();
     double update_vars();
     double m_originalScale;
     bool m_unset;