Normalize use of DISABLE_SCALERS. Now it really disables all scalers

except essential Normal1x. Also thumbnails get compiled unconditionally
so we get them on all platforms, even those which never display them.

Sorted out mess brought to sdl backend by epoc maintainer. He misused
__SYMBIAN32__ where DISABLE_SCALERS is more appropriate.

svn-id: r18850

common/scaler.cpp

@@ -147,6 +147,7 @@ void Normal1x(const uint8 *srcPtr, uint32 srcPitch, uint8 *dstPtr, uint32 dstPit
 	}
 }
 
+#ifndef DISABLE_SCALERS
 /**
  * Trivial nearest-neighbour 2x scaler.
  */
@@ -316,3 +317,5 @@ void DotMatrix(const uint8 *srcPtr, uint32 srcPitch, uint8 *dstPtr, uint32 dstPi
 		q += nextlineDst << 1;
 	}
 }
+
+#endif