Fix outline shader issue by pre-calculate some params.

[andy840119]

lowp vec4 outline(sampler2D tex, int radius, mediump vec2 texCoord, mediump vec2 texSize, mediump vec4 colour)
{
	float angle = 0.0;
	float outlineAlpha = 0.0;
	
	for (int i = 0; i < SAMPLES; i++)
	{
		angle += 1.0 / (float(SAMPLES) / 2.0) * PI;

		// todo: might need to adjust step samples amount to fill the inner side.
		// but it will cause lots of performance issue if make step samples larger.
		// so should find a better algorithm to fill inner colour.
		for (int j = 1; j <= STEP_SAMPLES; j++)
		{
			vec2 testPoint = texCoord - vec2(sin(angle), cos(angle)) * (float(radius) * (1.0 / j)) / texSize;
			float sampledAlpha = texture2D(tex, testPoint).a;
			outlineAlpha = max(outlineAlpha, sampledAlpha);
		}
	}

	mediump vec4 ogCol = texture2D(tex, texCoord);
	vec4 outlineCol = mix(vec4(0.0), colour, outlineAlpha);

	return mix(outlineCol, ogCol, ogCol.a);
}

Here's the shader and it can see that sin(angle), cos(angle)) runs lots of time on every frame (maybe run draw_width * draw_height * SAMPLES(2) * STEP_SAMPLES(128) times in each frame)
Maybe it can be calculated at the beginning.
Not really sure has someting like void init(void) before void main(void) and only run one time.

[andy840119]

also should think another issue:
should outline be transparent if source is transparent image also?
Because it might be faster if not doing the remaining work if already get the colour from texture2D.

[andy840119]

Also there's another things i cannot understand
How did osu-framework can make outline-link effect with sh_Blur.ts

Here's the demo:

// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
// See the LICENCE file in the repository root for full licence text.

using NUnit.Framework;
using osu.Framework.Graphics;
using osu.Framework.Graphics.Effects;
using osu.Framework.Graphics.Shapes;
using osu.Framework.Graphics.Sprites;
using osu.Framework.Testing;
using osuTK;
using osuTK.Graphics;

namespace osu.Framework.Tests.Visual
{
    public class OutlineTestScene : TestScene
    {
        [Test]
        public void Test()
        {
            AddStep("Apply shader", () =>
            {
                Add(new Box
                {
                    RelativeSizeAxes = Axes.Both,
                    Colour = Color4.Red
                });

                Add(new SpriteText
                {
                    Text = "Outlined Text",
                    Font = new FontUsage(size: 72),
                }.WithEffect(new OutlineEffect
                {
                    BlurSigma = new Vector2(5f),
                    Strength = 100f,
                    Colour = Color4.Blue,
                    PadExtent = true,
                }));
            });
        }
    }
}