renderer/opengl: Extract shaders from source (#9600)

---------

Co-authored-by: Mihai Fufezan <mihai@fufexan.net>

src/render/shaders/glsl/blurfinish_legacy.frag

@@ -0,0 +1,28 @@
+precision         highp float;
+varying vec2      v_texcoord; // is in 0-1
+uniform sampler2D tex;
+
+uniform float     noise;
+uniform float     brightness;
+
+float hash(vec2 p) {
+    vec3 p3 = fract(vec3(p.xyx) * 1689.1984);
+    p3 += dot(p3, p3.yzx + 33.33);
+    return fract((p3.x + p3.y) * p3.z);
+}
+
+void main() {
+    vec4 pixColor = texture2D(tex, v_texcoord);
+
+    // noise
+    float noiseHash   = hash(v_texcoord);
+    float noiseAmount = (mod(noiseHash, 1.0) - 0.5);
+    pixColor.rgb += noiseAmount * noise;
+
+    // brightness
+    if (brightness < 1.0) {
+        pixColor.rgb *= brightness;
+    }
+
+    gl_FragColor = pixColor;
+}