Fix buffer size calculations in lightmapper_rd.cpp to account for element sizes

[Meowcat285]

Fixes an issue where the buffer size passed into storage_buffer_create didn't equal the actual size of the data.

Fixes #107805

I'm not 100% sure if this is the best way to go about fixing this, but it worked for me at least. Let me know if there's a better way to do this

• Maintainer edit: Fixes regression from Use raw buffer pointers in RenderingDevice allocation APIs to avoid intermediary arrays #107486

[Ivorforce]

Ohh, I totally should have caught that. Thanks for finding and fixing!
I just have some suggestions for to avoid the later allocations :)

[Meowcat285]

@Ivorforce I applied all of your suggestions, thanks!

[Ivorforce]

I'm approving assuming the fix still works after the changes. Code looks good to me.

Before this can be merged, please squash your commits down into one. You can read up on the workflow here: https://docs.godotengine.org/en/stable/contributing/workflow/pr_workflow.html

[Meowcat285]

@Ivorforce squash is all set, but let me double check just to be sure that it still works before you merge it

[Meowcat285]

@Ivorforce Yep works perfect, ready to merge ✔️

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉