render/gles2: skip glslang check when shaders are unchanged

Currently, the glslang check is run every time ninja is invoked,
even with an up-to-date build directory when GLSL shaders haven't
been modified. This is due to glslang not creating any output
file: the _check file never exists so ninja keeps trying to
generate it by running the command.

Unfortunately Meson doesn't support running commands with no
outputs [1]. Create an empty output file to fix this by setting
`capture: true`.

This doesn't work out-of-the-box, because glslang prints messages
to stdout, and provides no way to change this [2]. As a result,
shader errors are not surfaced back to the user - they end up in
the _check file. Workaround this with a thin wrapper which
redirects stdout to stderr when invoking glslang.

[1]: https://github.com/mesonbuild/meson/issues/11506
[2]: https://github.com/KhronosGroup/glslang/pull/4138

render/gles2/shaders/check.sh

@@ -0,0 +1,4 @@
+#!/bin/sh -eu
+
+# glslang prints log messages to stdout, remap to stderr
+exec "$@" >&2

render/gles2/shaders/meson.build

@@ -1,3 +1,4 @@
+check = find_program('./check.sh', native: true)
 embed = find_program('./embed.sh', native: true)
 
 shaders = [
@@ -13,7 +14,8 @@ foreach name : shaders
 		'gles2-' + name,
 		input: name,
 		output: name + '_check',
-		command: [glslang, '@INPUT@'],
+		command: [check, glslang, '@INPUT@'],
+		capture: true,
 		build_by_default: true,
 	)