shader_prepass example: disable MSAA for maximum compatibility

[mockersf]

Objective

Since #8446, example shader_prepass logs the following error on my mac m1:

ERROR bevy_render::render_resource::pipeline_cache: failed to process shader:
error: Entry point fragment at Fragment is invalid
 = Argument 1 varying error
 = Capability MULTISAMPLED_SHADING is not supported

The example display the 3d scene but doesn't change with the preps selected

Maybe related to this update in naga: gfx-rs/naga@cc3a8ac

Solution

• Disable MSAA in the example, and check if it's enabled in the shader

[IceSentry]

A note explaining the ifdef in the shader itself might be nice too, but not necessary