Avoid detaching containers in range-based for loops

Static analysis flagged the glTF parser for iterating directly over the
result of QJsonValue::toArray(). The temporary array is non-const and
shares its data with the parsed document, so calling begin() on it
detaches and deep copies the array. Hold each array in a const local
before iterating instead.

The glTF mesh builder iterated a non-const local QVector of split
vertices, so wrap it in std::as_const.

Axivion-Id: qt_Add-ons_qtquick3d_dev_linux:SV1088
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Axivion-Id: qt_Add-ons_qtquick3d_dev_linux:SV1101
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Axivion-Id: qt_Add-ons_qtquick3d_dev_linux:SV1103
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Axivion-Id: qt_Add-ons_qtquick3d_dev_linux:SV1110
Axivion-Id: qt_Add-ons_qtquick3d_dev_linux:SV1111
Axivion-Id: qt_Add-ons_qtquick3d_dev_linux:SV1112
Axivion-Id: qt_Add-ons_qtquick3d_dev_linux:SV1113
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Axivion-Id: qt_Add-ons_qtquick3d_dev_linux:SV1117
Axivion-Id: qt_Add-ons_qtquick3d_dev_linux:SV1118
Change-Id: I7e0f34d9d796544488788ae7a9e491dfbf9b46b5
Reviewed-by: Christian Strømme <christian.stromme@qt.io>