Warn when bevy_sprite and bevy_pbr are not enabled with bevy_gizmos

[BD103]

Objective

• bevy_gizmos cannot work if both bevy_sprite and bevy_pbr are disabled.
• It silently fails to render, making it difficult to debug.
• Fixes Using bevy_gizmos feature with default-features=false renders nothing (requires extra bevy_pbr feature) #10984

Solution

• Log an error message when GizmoPlugin is registered.

Alternatives

I chose to log an error message, since it seemed the least intrusive of potential solutions. Some alternatives include:

• Choosing one dependency as the default, neglecting the other. (Add bevy_pbr as a dependency to bevy_gizmos #11035)
• Raising a compile error when neither dependency is enabled. (See my original comment)
• Raising a compile warning using a macro hack. (Pre-RFC - Add compile_warning! macro)
• Logging a warning instead of an error.
  • This might be the better option. Let me know if I should change it.

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Changelog

• bevy_gizmos will now log an error if neither bevy_pbr nor bevy_sprite are enabled.

[BD103]

I tested this commit using this Gist.

[doonv]

I like this. still have a few minor nitpicks tho

[alice-i-cecile]

This is an example of the problems caused by the lack of plugin dependencies (#69). I'm not sure we'll land that this cycle, despite the hard work of @NthTensor, so I'm alright to merge this for now.