gdk: Move GLSL shaders into GResource

Having the shaders inlined as C strings makes them harder to edit and
maintain.

gdk/resources/glsl/gl3-texture-2d.vs.glsl

@@ -0,0 +1,13 @@
+#version 150
+
+uniform sampler2D map;
+
+in vec2 position;
+in vec2 uv;
+
+out vec2 vUv;
+
+void main() {
+  gl_Position = vec4(position, 0, 1);
+  vUv = uv;
+}