meson: explicitly test for glad2 version

Building fails if the user only has glad1 installed, but not glad2, with
this error: argument --api: Invalid api-string: "gl:core,gles2,egl"

And if the user has neither glad1 nor glad2, it just fails with the
unhelpful "module not found". Improve both by explicitly testing for
this in the build system.

src/include/glad/meson.build

@@ -1,3 +1,18 @@
+glad_check = run_command([ python, '-c', 'import glad; print(glad.__version__)' ],
+  env: python_env,
+  capture: true,
+  check: false,
+)
+
+glad_ver = glad_check.returncode() == 0 ? glad_check.stdout().strip() : 'none'
+glad_req = '>= 2.0'
+
+if not glad_ver.version_compare(glad_req)
+  error(f'glad (required: @glad_req@, found: @glad_ver@) was not found in ' +
+        'PYTHONPATH or `3rdparty`. Please run `git submodule update --init` ' +
+        'followed by `meson --wipe`.')
+endif
+
 glad = custom_target('gl.h',
   output: 'gl.h',
   env: python_env,