fix example lightmaps after color migration

[mockersf]

Objective

• Since Migrate from LegacyColor to bevy_color::Color #12163 example lightmaps is more dull

Solution

• Use a srgba color, as it was before:
  

  bevy/examples/3d/lightmaps.rs

  Line 39 in b24ab2e

  materials.get_mut(material).unwrap().emissive = LegacyColor::WHITE * exposure;

  
  

  bevy/crates/bevy_render/src/color/mod.rs

  Line 132 in b24ab2e

  pub const WHITE: LegacyColor = LegacyColor::rgb(1.0, 1.0, 1.0);

[alice-i-cecile]

Hmm, this is an interesting case where the gamma correction of Srgba is useful. Good catch.

We should impl Div and the reciprocal Mul in this PR too however.

[pablo-lua]

I like the Mul and Div implementation, and the example is better now.