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>

src/gui/painting/qtextureglyphcache.cpp

@@ -285,6 +285,8 @@ QImageTextureGlyphCache::~QImageTextureGlyphCache()
 void QImageTextureGlyphCache::resizeTextureData(int width, int height)
 {
     m_image = m_image.copy(0, 0, width, height);
+    // Regions not part of the copy are initialized to 0, and that is just what
+    // we need.
 }
 
 void QImageTextureGlyphCache::createTextureData(int width, int height)
@@ -305,6 +307,12 @@ void QImageTextureGlyphCache::createTextureData(int width, int height)
     default:
         Q_UNREACHABLE();
     }
+
+    // Regions not touched by the glyphs must be initialized to 0. (such
+    // locations may in fact be sampled with styled (shifted) text materials)
+    // When resizing, the QImage copy() does this implicitly but the initial
+    // contents must be zeroed out explicitly here.
+    m_image.fill(0);
 }
 
 void QImageTextureGlyphCache::fillTexture(const Coord &c, glyph_t g, QFixed subPixelPosition)