Gerber Viewer: put drills/outlines/keep outs on top by default

For file extension sorting, this priorities drills/outlines/KOs.

For X2 sorting, this changes the default priority to make
unknown-priority layers on top since if they contain anything that can
be plotted, it's probably drill data.

Fixes: https://gitlab.com/kicad/code/kicad/-/issues/10598

gerbview/X2_gerber_attributes.cpp

@@ -236,7 +236,7 @@ bool X2_ATTRIBUTE_FILEFUNCTION::IsDrillFile()
 
 void X2_ATTRIBUTE_FILEFUNCTION::set_Z_Order()
 {
-    m_z_order = -100;     // low level
+    m_z_order = 100;     // high level
     m_z_sub_order = 0;
 
     if( IsCopper() )

gerbview/gerber_file_image_list.cpp

@@ -228,6 +228,7 @@ static struct GERBER_ORDER gerberFileExtensionOrder[] =
        { ".XLN",        GERBER_ORDER_ENUM::GERBER_DRILL },
        { ".TAP",        GERBER_ORDER_ENUM::GERBER_DRILL },
        { ".DRD",        GERBER_ORDER_ENUM::GERBER_DRILL },
+       { ".DRL",        GERBER_ORDER_ENUM::GERBER_DRILL },
        { ".NC",         GERBER_ORDER_ENUM::GERBER_DRILL },
        { ".XNC",        GERBER_ORDER_ENUM::GERBER_DRILL },
 

gerbview/gerber_file_image_list.h

@@ -35,7 +35,11 @@
 
 enum class GERBER_ORDER_ENUM : int
 {
-    GERBER_TOP_PASTE = 0,
+    GERBER_DRILL = 0,
+    GERBER_BOARD_OUTLINE,
+    GERBER_KEEP_OUT,
+    GERBER_MECHANICAL,
+    GERBER_TOP_PASTE,
     GERBER_TOP_SILK_SCREEN,
     GERBER_TOP_SOLDER_MASK,
     GERBER_TOP_COPPER,
@@ -44,10 +48,6 @@ enum class GERBER_ORDER_ENUM : int
     GERBER_BOTTOM_SOLDER_MASK,
     GERBER_BOTTOM_SILK_SCREEN,
     GERBER_BOTTOM_PASTE,
-    GERBER_KEEP_OUT,
-    GERBER_MECHANICAL,
-    GERBER_BOARD_OUTLINE,
-    GERBER_DRILL,
     GERBER_LAYER_UNKNOWN,
 };