Avoid uninitialized texture data in image glyph cache

The problem becomes visible with styled native text materials in Quick,
but only in certain cases: the regions not used by glyphs in the QImage
are undefined (if they are 0 there's no problem) - but the whole
code path is only used when the fbo readback workaround is enabled.

When these conditions met, the styled text materials may sample locations
with uninitialized data in the texture, showing small artifacts around
the glyphs when shifting is involved in the styling.

The non-image based GL glyph cache handles this by an explicit upload
with all 0's when creating the texture - the QImage code path should do
the same then.

Change-Id: I818ee19f87c6a147e42cd3ead39645da4d0fef11
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

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