Issue #1354 - Don't allow glsl[130,400] unless we have gpu_shader5

- Teach GLContext about gpu_shader5
- Downgrade shader language version if gpu_shader5 support isn't found.

dom/canvas/WebGLShaderValidator.cpp

@@ -5,6 +5,7 @@
 
 #include "WebGLShaderValidator.h"
 
+#include <algorithm>
 #include "angle/ShaderLang.h"
 #include "gfxPrefs.h"
 #include "GLContext.h"
@@ -112,6 +113,16 @@ ShaderOutput(gl::GLContext* gl)
         return SH_ESSL_OUTPUT;
     } else {
         uint32_t version = gl->ShadingLanguageVersion();
+
+        // Version 130 starts to require integral constant expressions for loop indices,
+        // instead of "constant-index-expression".
+        // Both version 400 and gpu_shader5 remove this restrictions.
+        // gpu_shader5 went core in 400, so we can just check for the GLFeature.
+        // If we're compiling for webglsl1, even for webgl2, we need gpu_shader5, or GLSL_COMPAT.
+        if (!gl->IsSupported(gl::GLFeature::gpu_shader5)) {
+            version = std::min<uint32_t>(version, 120);
+        }
+
         switch (version) {
         case 100: return SH_GLSL_COMPATIBILITY_OUTPUT;
         case 120: return SH_GLSL_COMPATIBILITY_OUTPUT;