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.

meson.build

@@ -3,7 +3,7 @@ project(
 	'c',
 	version: '0.16.0-dev',
 	license: 'MIT',
-	meson_version: '>=0.58.1',
+	meson_version: '>=0.59.0',
 	default_options: [
 		'c_std=c11',
 		'warning_level=2',