Handle missing mixins and applying mixin attributes to mappings of primitives

[mrxz]

Description:
Primitives handled mixins differently compared to a-node. In case a mixin was referenced that didn't exist, the primitive would throw an error while the a-node silently ignored it. Now a warning is logged in both cases. Primitives now also check if attributes on mixins correspond to any of their mappings, allowing things like the following to work:

<a-assets>
    <a-mixin id="red" color="red"></a-mixin>
</a-assets>

<a-box mixin="red"></a-box>

Fixes #4455
Fixes #4717

Changes proposed:

• Log warning when mixin can't be found (instead of throwing an error or silently continuing).
• Apply mapping in primitive for any mixin attributes that correspond with defined mappings.

[dmarcos]

Thanks!