Fix windows build error and warnings

Pull Request: https://projects.blender.org/blender/blender/pulls/135567

source/blender/editors/space_sequencer/sequencer_intern.hh

@@ -45,8 +45,8 @@ struct ListBase;
 
 namespace blender::ed::vse {
 
-struct SeqQuadsBatch;
-struct StripsDrawBatch;
+class SeqQuadsBatch;
+class StripsDrawBatch;
 
 #define DEFAULT_IMG_STRIP_LENGTH 25 /* XXX arbitrary but ok for now. */
 

source/blender/sequencer/SEQ_offscreen.hh

@@ -14,8 +14,12 @@
 #include "IMB_imbuf_types.hh"
 
 struct GPUOffScreen;
-
+struct GPUViewport;
+struct Depsgraph;
+struct View3DShading;
+struct Object;
 enum eDrawType;
+enum eV3DOffscreenDrawFlag;
 
 namespace blender::seq {
 typedef struct ImBuf *(*SequencerDrawView)(struct Depsgraph *depsgraph,