[Fix] GltfExporter correctly handles multiple mesh instances per entity

[mvaligursky]

• before, we would walk all mesh instances in a render component, and store their index as a mesh index of a node. The problem was that if there were multiple mesh instances, the index would get overwritten and only the last mesh would be assigned
• this PR changes the way mesh instances are collected to collect them per node. And from all of these, a single gltf mesh is created, with each mesh instance forming a primitive.
• this also makes export of model components working - as all mesh instances are attached to a root node, even though internally they store the node they need the transform from.

In the screenshot, both a sphere and a cone are added to a single render component, and both meshes are exported. Before, only the cone (second mesh) would be visible.