render/gles2: move shaders to individual files

Instead of having a C file with strings for each shader, move each
shader into its own file. Use a small POSIX shell script to convert
the files into C strings (can't wait for C23 #embed...).

The benefits from this are:

- Improved readability and syntax highlighting.
- Line numbers in shader compiler errors are easier to make sense of.
- Consistency with the Vulkan renderer.
- Shaders will become more complicated as we add color management
  features.

render/gles2/shaders/tex_external.frag

@@ -0,0 +1,10 @@
+#extension GL_OES_EGL_image_external : require
+
+precision mediump float;
+varying vec2 v_texcoord;
+uniform samplerExternalOES texture0;
+uniform float alpha;
+
+void main() {
+	gl_FragColor = texture2D(texture0, v_texcoord) * alpha;
+}