GLSL: Fix post-depth coverage for ESSL.

ESSL does not support `GL_ARB_post_depth_coverage`. There, we must use
`GL_EXT_post_depth_coverage`. I've added this as a fallback for desktop
as well.

Note that `GL_EXT_post_depth_coverage` also requires the fragment shader
to set `early_fragment_tests` explicitly, while
`GL_ARB_post_depth_coverage` does not. It doesn't really matter either
way, since `SPV_KHR_post_depth_coverage` *also* requires both execution
modes to be explicitly set.

1 files changed, 13 insertions, 0 deletions

diff --git a/shaders/frag/post-depth-coverage-es.frag b/shaders/frag/post-depth-coverage-es.frag
new file mode 100644
index 00000000..ecc57e4a
--- /dev/null
+++ b/shaders/frag/post-depth-coverage-es.frag
@@ -0,0 +1,13 @@
+#version 310 es
+#extension GL_EXT_post_depth_coverage : require
+#extension GL_OES_sample_variables : require
+precision mediump float;
+
+layout(early_fragment_tests, post_depth_coverage) in;
+
+layout(location = 0) out vec4 FragColor;
+
+void main()
+{
+	FragColor = vec4(gl_SampleMaskIn[0]);
+}