gpu: fix syntax issue in shader.

The “f” suffix is valid GLSL, but WebGL does not like it so let’s disable it.

src/gpu/palette.lolfx

@@ -33,7 +33,7 @@ varying vec2 pass_TexCoord;
 void main()
 {
     vec4 pal = texture2D(u_texture, pass_TexCoord);
-    vec4 col = texture2D(u_palette, vec2(pal.x, 0.f));
+    vec4 col = texture2D(u_palette, vec2(pal.x, 0.0));
     if (pal.x == 0.0)
         discard;
     gl_FragColor = col;