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

---------

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

src/render/shaders/glsl/quad.frag

@@ -0,0 +1,14 @@
+#extension GL_ARB_shading_language_include : enable
+precision highp float;
+varying vec4 v_color;
+
+#include "rounding.glsl"
+
+void main() {
+    vec4 pixColor = v_color;
+
+    if (radius > 0.0) 
+        pixColor = rounding(pixColor);
+
+    gl_FragColor = pixColor;
+}