[TextServer] Improve ligature cursor handling.

Fix mid-grapheme hit test.
Fix OpenType features property handling, add default features override option.
Enable mid-grapheme cursor by default.

doc/classes/TextServerExtension.xml

@@ -261,6 +261,13 @@
 				Returns font family name.
 			</description>
 		</method>
+		<method name="_font_get_opentype_feature_overrides" qualifiers="virtual const">
+			<return type="Dictionary" />
+			<argument index="0" name="font_rid" type="RID" />
+			<description>
+				Returns font OpenType feature set override.
+			</description>
+		</method>
 		<method name="_font_get_oversampling" qualifiers="virtual const">
 			<return type="float" />
 			<argument index="0" name="font_rid" type="RID" />
@@ -550,6 +557,7 @@
 			<argument index="0" name="font_rid" type="RID" />
 			<argument index="1" name="force_autohinter" type="bool" />
 			<description>
+				If set to [code]true[/code] auto-hinting is preffered over font built-in hinting.
 			</description>
 		</method>
 		<method name="_font_set_global_oversampling" qualifiers="virtual">
@@ -670,6 +678,14 @@
 				Sets the font family name.
 			</description>
 		</method>
+		<method name="_font_set_opentype_feature_overrides" qualifiers="virtual">
+			<return type="void" />
+			<argument index="0" name="font_rid" type="RID" />
+			<argument index="1" name="overrides" type="Dictionary" />
+			<description>
+				Sets font OpenType feature set override.
+			</description>
+		</method>
 		<method name="_font_set_oversampling" qualifiers="virtual">
 			<return type="void" />
 			<argument index="0" name="font_rid" type="RID" />