Reintroduce color fixes to WebGL1 backend, but only if sRGB textures are supported (#652)

Also removed the 300es shaders, as they'd be identical to the 100es ones.

egui_demo_lib/src/apps/color_test.rs

@@ -39,7 +39,7 @@ impl epi::App for ColorTest {
             if frame.is_web() {
                 ui.colored_label(
                     RED,
-                    "NOTE: The WebGL1 backend does NOT pass the color test. The WebGL2 backend does."
+                    "NOTE: The WebGL1 backend without sRGB support does NOT pass the color test.",
                 );
                 ui.separator();
             }