Initial implementation of NewGRF custom signal styles

docs/newgrf-additions-nml.html

@@ -445,6 +445,26 @@ item (FEAT_GLOBALVARS) {
 			N.B. Realistic braking must be enabled for additional signal aspects to be used
 		</td>
 	</tr>
+	<tr><td>define_style</td><td>1 - 255</td>
+		<td>
+			Define a custom signal style<br />
+			Signals using this style will only use this GRF, or the default graphics if no graphics are returned.<br />
+			The value supplied is returned in the signal_style variable.<br />
+			This property (and related signal style properties) may be used more than once.<br />
+			The total number of custom signal styles in a game is currently limited to 15.
+		</td>
+	</tr>
+	<tr><td>style_name</td><td>string</td>
+		<td>
+			Set the name of the most recently defined style (defined using the define_style property).<br />
+			This property should be used if using the define_style property, as otherwise the style will have no name.
+		</td>
+	</tr>
+	<tr><td>no_default_style</td><td>0 or 1</td>
+		<td>
+			When enabled, custom signal graphics from this GRF are only used for custom signal styles, not the default style
+		</td>
+	</tr>
 	</table>
 	<br />
 	<table>
@@ -489,6 +509,11 @@ item (FEAT_GLOBALVARS) {
 			Above signal context variables in one variable (all of the signals_signal_context variable)
 		</td>
 	</tr>
+	<tr><td>signal_style</td><td>0 - 255</td>
+		<td>
+			The style ID defined using define_style for signals using a custom style, or 0 for signals using the default style
+		</td>
+	</tr>
 	</table>
 	<p>
 	Custom signal sprites example: