Tracking: Highlight keyframes in path visualization

This gives better idea of what's going on with your track. Example:

{F693806}

Color of keyframes are configurable from theme editor of clip editor.

Reviewers: keir, brecht, Severin

Differential Revision: https://developer.blender.org/D2772

source/blender/editors/interface/resources.c

@@ -779,6 +779,12 @@ const uchar *UI_ThemeGetColorPtr(bTheme *btheme, int spacetype, int colorid)
         case TH_PATH_AFTER:
           cp = ts->path_after;
           break;
+        case TH_PATH_KEYFRAME_BEFORE:
+          cp = ts->path_keyframe_before;
+          break;
+        case TH_PATH_KEYFRAME_AFTER:
+          cp = ts->path_keyframe_after;
+          break;
         case TH_CAMERA_PATH:
           cp = ts->camera_path;
           break;