finish CKBitmapData writer.

- finish spec fmt image writer and raw bitmap writer.
- fix a possible fatal issue in ReadBuffer(). return nullptr simply rather than allocate a blank buf. otherwise the process of use nullptr to check result will be broken.
- add CanSaveAlpha() method in CKBitmapHandler.

LibCmo/CK2/DataHandlers/CKBitmapHandler.cpp

@@ -256,6 +256,10 @@ namespace LibCmo::CK2::DataHandlers {
 			});
 	}
 
+	bool CKBitmapBMPHandler::CanSaveAlpha() {
+		return false;
+	}
+
 #pragma endregion
 
 #pragma region CKBitmapTGAHandler
@@ -293,6 +297,10 @@ namespace LibCmo::CK2::DataHandlers {
 			});
 	}
 
+	bool CKBitmapTGAHandler::CanSaveAlpha() {
+		return true;
+	}
+
 #pragma endregion
 	
 #pragma region General Getter Freer