Add some padding to the egui-wgpu uniform buffer for WebGL

[expenses]

I'm using egui-wgpu with the wgpu's WebGL backend. At the moment I get the following error on Chromium:

[.WebGL-0x38140334e800] GL_INVALID_OPERATION: It is undefined behaviour to use a uniform buffer that is too small.

with nothing rendering. I've encountered this issue before and have added some padding to the UniformBuffer struct. There is now no warning and it renders normally.

[emilk]

cargo fmt is unhappy