gl_viewer skips primitive morph targets

[droettger]

Follow-up to #36

Using continue when the accessor has no buffer view index for TEXCOORD_0 is incorrect.
https://github.com/spnda/fastgltf/blob/main/examples/gl_viewer/gl_viewer.cpp#L423
The continue only works for the position attribute because that is strictly required.

For all other attributes that needs to be a runtime check, like in this case if (texCoordAccessor.bufferViewIndex.has_value()) to just skip the VAO 1 setting.
The per mesh glVertexArrayElementBuffer at the end must still be done unconditionally or OpenGL will complain about an invalid input data types.

[spnda]

The check is whether the UV accessor has a buffer view associated with it. The example doesn't support sparse accessors at all currently. but I guess I could fix that fairly quickly by refactoring the code to use fastgltf::copyFromAccessor into some GL buffer. However, I wanted to keep the example fairly simple, so currently it just doesn't support sparse accessors and I honestly don't feel like completely refactoring how the example works for adding sparse accessor support. I'll leave this open as a reminder though.

[spnda]

This was fixed recently with 73673cc.