render/gles2: check shaders once

We'd always check shaders each time we were building, even if the
shaders didn't change. This is caused by the check not producing
any file, so ninja can't check whether the output file is up-to-date.

Fix this by capturing the output and write to a file.

TODO: error messages are no longer visible.

render/gles2/shaders/meson.build

@@ -14,6 +14,7 @@ foreach name : shaders
 		input: name,
 		output: name + '_check',
 		command: [glslang, '@INPUT@'],
+		capture: true,
 		build_by_default: true,
 	)