Add Gamma correction in palette editor

Particularly useful when working for platforms with non-linear palettes.
For example the Thomson TO8 has a Gamma correction of 2.8, so the
darkest shade of color is already very bright. It is important to take
this into account when drawing pictures to avoid bad surprises when
showing them on the real machine.

The setting is available from the command line, or from the extended
palette window, which gets a little relayout using horizontal sliders
now that these are available.

src/main.c

@@ -107,7 +107,8 @@ void Display_syntax(void)
   printf("\t-tall2            to emulate a video mode with double tall pixels (2x4)\n");
   printf("\t-triple           to emulate a video mode with triple pixels (3x3)\n");
   printf("\t-quadruple        to emulate a video mode with quadruple pixels (4x4)\n");
-  printf("\t-rgb n            to reduce RGB precision from 256 to n levels\n");
+  printf("\t-rgb n            to reduce RGB precision (2 to 256, 256=max precision)\n");
+  printf("\t-gamma n          to adjust Gamma correction (1 to 30, 10=no correction)\n");
   printf("\t-skin <filename>  to use an alternate file with the menu graphics\n");
   printf("\t-mode <videomode> to set a video mode\n");
   printf("Arguments can be prefixed either by / - or --\n");
@@ -208,6 +209,7 @@ enum CMD_PARAMS
     CMDPARAM_PIXELRATIO_TALL3,
     CMDPARAM_PIXELRATIO_WIDE2,
     CMDPARAM_RGB,
+    CMDPARAM_GAMMA,
     CMDPARAM_SKIN
 };
 
@@ -229,6 +231,7 @@ struct {
     {"tall3", CMDPARAM_PIXELRATIO_TALL3},
     {"wide2", CMDPARAM_PIXELRATIO_WIDE2},
     {"rgb", CMDPARAM_RGB},
+    {"gamma", CMDPARAM_GAMMA},
     {"skin", CMDPARAM_SKIN}
 };
 
@@ -337,7 +340,7 @@ int Analyze_command_line(int argc, char * argv[], char *main_filename, char *mai
         Pixel_ratio = PIXEL_WIDE2;
         break;
       case CMDPARAM_RGB:
-        // echelle des composants RGB
+        /* RGB scale */
         index++;
         if (index<argc)
         {
@@ -358,6 +361,28 @@ int Analyze_command_line(int argc, char * argv[], char *main_filename, char *mai
           exit(0);
         }
         break;
+      case CMDPARAM_GAMMA:
+        /* Gamma correction */
+        index++;
+        if (index<argc)
+        {
+          int scale;
+          scale = atoi(argv[index]);
+          if (scale < 1 || scale > 30)
+          {
+            Error(ERROR_COMMAND_LINE);
+            Display_syntax();
+            exit(0);
+          }
+          Set_palette_Gamma(scale);
+        }
+        else
+        {
+          Error(ERROR_COMMAND_LINE);
+          Display_syntax();
+          exit(0);
+        }
+        break;
       case CMDPARAM_SKIN:
         // GUI skin file
         index++;