The shadows' textures cannot be prevented from generating

## Bevy version

0.13.2

## What you did

Set the shadows_enabled property of PointLight to false

## What went wrong

The shadow texture of PointLight still occupy video memory. I haven't even added DirectionalLight, yet its shadow texture are also taking up video memory.

If I set the size of PointLightShadowMap to 0, the program crashes. Therefore, I have to set it to 1 to reduce memory usage.

Start by tracing PointLight shadow handling and PointLightShadowMap allocation in the rendering code, reproducing the case with shadows_enabled set to false. Verify the behavior for both disabled point-light shadows and a shadow-map size of 0; done means unused shadow textures no longer consume video memory and size 0 does not crash.

Written by the indexing model from the issue text.