Fix issues while rendering RTL text in Text Visual

This patch fixes two issue:

1. Overlapped glyph is cut.
2. Small gap between joint glyphs.

The solution is:

1. When we copy glyphs to the big image buffer, we should only copy
   non-transparent pixels, and this will prevent overlapped glyphs
   from being cut.

2. For any pixel overlapped with the pixel in previous glyphs,
   make sure we don't overwrite a previous bigger alpha with a smaller
   alpha, so that we can avoid semi-transparent gaps between joint
   glyphs with overlapped pixels, which could happen e.g. in the RTL
   text when we copy glyphs from right to left.

Change-Id: Id4d9d028999612e0545aa47b5bf66e410fc7ee91