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/embed.sh

@@ -0,0 +1,11 @@
+#!/bin/sh -eu
+
+var=${1:-data}
+hex="$(od -A n -t x1 -v)"
+
+echo "static const char $var[] = {"
+for byte in $hex; do
+	echo "	0x$byte,"
+done
+echo "	0x00,"
+echo "};"