egui_web: always use the glow painter, and remove the old WebGL code. (#1356)

* egui_web: always use the glow painter, and remove the old WebGL code.
* Clean up the WebPainter trait
* Clarify WebGL1 warning text in color test

The glow painter became standard in egui 0.17, and I've heard no complaints! So let's simplify and go all in on glow.

Part of https://github.com/emilk/egui/issues/1198

egui_web/Cargo.toml

@@ -27,18 +27,12 @@ crate-type = ["cdylib", "rlib"]
 
 
 [features]
-default = ["default_fonts", "glow"]
+default = ["default_fonts"]
 
 # If set, egui will use `include_bytes!` to bundle some fonts.
 # If you plan on specifying your own fonts you may disable this feature.
 default_fonts = ["egui/default_fonts"]
 
-# Use glow as the renderer.
-glow = ["egui_glow", "egui_glow/epi"]
-
-# Alternative to the glow renderer.
-webgl = []
-
 # enable persisting egui memory
 persistence = ["egui/persistence", "ron", "serde"]
 
@@ -52,7 +46,7 @@ egui = { version = "0.17.0", path = "../egui", default-features = false, feature
   "single_threaded",
   "tracing",
 ] }
-egui_glow = { version = "0.17.0", path = "../egui_glow", optional = true, default-features = false }
+egui_glow = { version = "0.17.0", path = "../egui_glow", default-features = false }
 epi = { version = "0.17.0", path = "../epi" }
 
 bytemuck = "1.7"
@@ -106,15 +100,8 @@ features = [
   "TouchEvent",
   "TouchList",
   "WebGl2RenderingContext",
-  "WebGlBuffer",
   "WebglDebugRendererInfo",
-  "WebGlFramebuffer",
-  "WebGlProgram",
   "WebGlRenderingContext",
-  "WebGlShader",
-  "WebGlTexture",
-  "WebGlUniformLocation",
-  "WebGlVertexArrayObject",
   "WheelEvent",
   "Window",
 ]