Text: suggest using the returned range for glyph cache image flush.

Now it's less code, and it also no longer results in random edge
artifacts because the padding area wasn't correctly uploaded. I'm going
to do the same change in the FreeTypeFont and StbTrueTypeFont next.

doc/snippets/Text.cpp

@@ -199,15 +199,15 @@ Containers::Array<Vector2i> offsets{NoInit, images.size()};
 cache.atlas().clearFlags(
     TextureTools::AtlasLandfillFlag::RotatePortrait|
     TextureTools::AtlasLandfillFlag::RotateLandscape);
-CORRADE_INTERNAL_ASSERT(cache.atlas().add(
+Containers::Optional<Range2Di> range = cache.atlas().add(
     stridedArrayView(images).slice(&ImageView2D::size),
-    offsets));
+    offsets);
+CORRADE_INTERNAL_ASSERT(range);
 /* [AbstractGlyphCache-filling-atlas] */
 
 /* [AbstractGlyphCache-filling-glyphs] */
 /* The glyph cache is just 2D, so copying to the first slice */
 Containers::StridedArrayView3D<char> dst = cache.image().pixels()[0];
-Range2Di updated;
 for(UnsignedInt i = 0; i != images.size(); ++i) {
     Range2Di rectangle = Range2Di::fromSize(offsets[i], images[i].size());
     cache.addGlyph(fontId, i, {}, rectangle);
@@ -218,13 +218,10 @@ for(UnsignedInt i = 0; i != images.size(); ++i) {
         std::size_t(offsets[i].y()),
         std::size_t(offsets[i].x()),
         0}, src.size()));
-
-    /* Maintain a range that was updated in the glyph cache */
-    updated = Math::join(updated, rectangle);
 }
 
-/* Reflect the image data update to the actual GPU-side texture */
-cache.flushImage(updated);
+/* Reflect the updated image range to the actual GPU-side texture */
+cache.flushImage(*range);
 /* [AbstractGlyphCache-filling-glyphs] */
 }
 

src/Magnum/Text/AbstractGlyphCache.h

@@ -166,15 +166,17 @@ efficiency.
 
 @snippet Text.cpp AbstractGlyphCache-filling-atlas
 
-In case the layouting fails, triggering the assertion, the cache size was
-picked too small or there was already enough glyphs added that the new ones
-didn't fit. The solution is then to either increase the cache size, turn the
-cache into an array, or create a second cache for the new data. Depending on
-the input sizes it's also possible to achieve a better packing efficiency by
-toggling various @ref TextureTools::AtlasLandfillFlag values --- you can for
-example create temporary instances aside, attempt packing with them, and then
-for filling the glyph cache itself pick the set of flags that resulted in the
-smallest @ref TextureTools::AtlasLandfill::filledSize().
+On success, @ref TextureTools::AtlasLandfill::add() returns a range spanning
+all added images, which we'll use later for GPU texture upload. In case of a
+failure, triggering the assertion above, the cache size was picked too small or
+there was already enough glyphs added that the new ones didn't fit. The
+solution is then to either increase the cache size, turn the cache into an
+array, or create a second cache for the new data. Depending on the input sizes
+it's also possible to achieve a better packing efficiency by toggling various
+@ref TextureTools::AtlasLandfillFlag values --- you can for example create
+temporary instances aside, attempt packing with them, and then for filling the
+glyph cache itself pick the set of flags that resulted in the smallest
+@ref TextureTools::AtlasLandfill::filledSize().
 
 @subsection Text-AbstractGlyphCache-filling-glyphs Adding glyphs