- 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

backend/tests/test_png.c

@@ -65,7 +65,7 @@ static void test_pixel_plot(const testCtx *const p_ctx) {
 
     const char *const have_identify = testUtilHaveIdentify();
 
-    testStartSymbol("test_pixel_plot", &symbol);
+    testStart("test_pixel_plot");
 
     for (i = 0; i < data_size; i++) {
         int size;