Carbide legacy shader support

[simsaens]

Feature description

There is no reason existing legacy shaders can't run in Carbide. They just have a different format that will need some massaging to work with Metal and BGFX

• Carbide could detect that an asset key refers to a legacy .shader bundle via that bundle's Info.plist
• Carbide could convert the shader to Metal using ShaderCross, as it does for other GLSL shaders
• The default shader uniforms will have to be rewritten, potentially at the AST level — this will ensure that they are BGFX compatible
• The API for accessing shader properties will need to work the same in Carbide on legacy shaders
• We should then be able to render legacy shaders with sprite, assign them to mesh.shader and use them elsewhere