Improve compiler compatibility (#743)

* fix returns

* use standard C headers

* struct/class mismatch

* explicit this in template

* switch variable scope

* C standard compliance

* & l-value

include/JSystem/J3DGraphBase/J3DShape.h

@@ -55,7 +55,7 @@ public:
 
 class J3DMaterial;
 class J3DVertexData;
-class J3DDrawMtxData;
+struct J3DDrawMtxData;
 
 enum J3DShpFlag {
     J3DShpFlag_Hide = 0x0001,

include/JSystem/J3DGraphBase/J3DTevs.h

@@ -204,7 +204,7 @@ struct J3DNBTScale : public J3DNBTScaleInfo {
     Vec* getScale() { return &mScale; }
 };
 
-class J3DTexCoord;
+struct J3DTexCoord;
 void loadTexCoordGens(u32, J3DTexCoord*);
 void loadNBTScale(J3DNBTScale& param_0);