Clarify normalized coordinates in SKGL

fixes #341

docs/docs/shaders/language.md

@@ -16,7 +16,10 @@ import {Skia} from "@shopify/react-native-skia";
 
 const source = Skia.RuntimeEffect.Make(`
 vec4 main(vec2 pos) {
-  vec2 normalized = pos/vec2(256.0, 256.0);
+  // The canvas is 256x256
+  vec2 canvas = vec2(256);
+  // normalized x,y values go from 0 to 1
+  vec2 normalized = pos/canvas;
   return vec4(normalized.x, normalized.y, 0.5, 1);
 }`);
 
@@ -43,6 +46,7 @@ import {Skia, Canvas, Paint, Shader, Fill} from "@shopify/react-native-skia";
 
 const source = Skia.RuntimeEffect.Make(`
 vec4 main(vec2 pos) {
+  // normalized x,y values go from 0 to 1, the canvas is 256x256
   vec2 normalized = pos/vec2(256);
   return vec4(normalized.x, normalized.y, 0.5, 1);
 }`)!;