- 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/extScale.ui

@@ -54,7 +54,7 @@
         <double>0.001000000000000</double>
        </property>
        <property name="maximum">
-        <double>100.000000000000000</double>
+        <double>10.000000000000000</double>
        </property>
        <property name="singleStep">
         <double>0.010000000000000</double>
@@ -140,7 +140,7 @@
         <string/>
        </property>
        <property name="maximum">
-        <number>9999</number>
+        <number>1000</number>
        </property>
        <property name="minimum">
         <number>1</number>