fix(rendering): Prevent crash in Render2DClass::Set_Texture and fix REF_PTR_SET self-assignment

[seer-by-sentry]

This PR addresses issue CLIENT-2RY, a recurring fatal crash in Render2DClass::Set_Texture.

Root Cause:
W3DDisplay::drawImage was passing a null pointer, obtained from image->getRawTextureData(), directly to Render2DClass::Set_Texture without a null check. This occurred when an image (e.g., for a text entry gadget) had no raw texture data, leading to an EXCEPTION_ACCESS_VIOLATION_WRITE.

Additionally, the REF_PTR_SET macro in refcount.h was identified as having a potential self-assignment bug (though less directly related to this specific crash, it was part of a previously unmerged fix).

Changes Implemented:

1. Null Check in W3DDisplay::drawImage: Added a null check for the tex pointer immediately after image->getRawTextureData() is called in both Generals/Code/GameEngineDevice/Source/W3DDevice/GameClient/W3DDisplay.cpp and GeneralsMD/Code/GameEngineDevice/Source/W3DDevice/GameClient/W3DDisplay.cpp. If tex is null, the function now returns early, preventing the crash.
2. REF_PTR_SET Self-Assignment Fix: Modified the REF_PTR_SET macro in Core/Libraries/Source/WWVegas/WWLib/refcount.h to include a if ((dst) != (src)) guard. This ensures that Add_Ref() and Release_Ref() are only called when assigning a different object, preventing potential issues with self-assignment and unnecessary reference count operations.

These fixes correspond to previously identified but unmerged PRs (#436 for the null check and #293 for the REF_PTR_SET macro).

Fixes CLIENT-2RY