CompositionObject: Set color palette to be transparent by default.

This fixes some PGS subtitles having opaque background instead of
transparent.
Based on commit by kasper93 @mpc-hc

src/subtitles/CompositionObject.cpp

@@ -31,7 +31,7 @@ CompositionObject::CompositionObject()
     m_nRLEDataSize  = 0;
     m_nRLEPos       = 0;
 
-    memsetd(m_Colors, 0xFF000000, sizeof(m_Colors));
+    memsetd(m_Colors, 0x00000000, sizeof(m_Colors));
 }
 
 CompositionObject::~CompositionObject()