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

---------

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

src/render/shaders/glsl/passthru.frag

@@ -0,0 +1,7 @@
+precision highp float;
+varying vec2 v_texcoord; // is in 0-1
+uniform sampler2D tex;
+
+void main() {
+    gl_FragColor = texture2D(tex, v_texcoord);
+}