Fix: images with background color now respects rounding

[vincent-sparks]

Code:

use egui::{TextureHandle, Image, Vec2, Layout, Direction};

struct App {
    texture: TextureHandle,
}

const ROW_HEIGHT: f32 = 200.0;

impl App {
    fn new(ctx: &eframe::CreationContext) -> Self {
        Self {
            texture: ctx.egui_ctx.load_texture("sample", egui::ColorImage::example(), egui::TextureOptions::default()),
        }
    }
}

impl eframe::App for App {
    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
        egui::CentralPanel::default().frame(egui::Frame::dark_canvas(&ctx.style())).show(ctx, |ui| {
            ui.with_layout(Layout::centered_and_justified(Direction::TopDown), |ui|{
                ui.add(Image::new(&self.texture)
                       .bg_fill(egui::Color32::WHITE)
                       .rounding(30.0)
                      );
            });
        });
    }
}

fn main() -> eframe::Result<()> {
    eframe::run_native("egui test", eframe::NativeOptions{
        viewport: egui::ViewportBuilder::default().with_inner_size((256.,200.)),
        ..Default::default()
    }, Box::new(|cc| Box::new(App::new(cc))))
}

Before:

After:

(please see the comment in the diff -- definitely not sure if it's up to code standards. you might want to change/remove that comment as well as the Mesh codepath if it was not, in fact, done for performance reasons)