Shaders: ah so this wasn't a Mesa bug, it was my stupidity.

I just put this aside when I discovered the error, thinking it was a
Mesa bug. Now that ARM Mali yelled about the same, I realized it wasn't
just Mesa.

Note to self: Mesa has no bugs. Can you just finally accept that?!

doc/changelog.dox

@@ -403,6 +403,9 @@ See also:
 -   For meshes with multiple sets of vertex attributes (such as texture
     coordinates), @ref MeshTools::compile() should be using only the first set
     but it wasn't.
+-   @ref Shaders::MeshVisualizerGL3D "Shaders::MeshVisualizerGL*D" shader
+    compilation failed with missing @glsl gl_PrimitiveID @ce due to GLSL 3.20
+    not being properly used for the @glsl #version @ce directive
 -   @ref Shaders::PhongGL was normalizing light direction in vertex shader,
     causing the fragment-interpolated direction being incorrect with visible
     artifacts on long polygons under low light angle

src/Magnum/Shaders/MeshVisualizerGL.cpp

@@ -137,7 +137,8 @@ GL::Version MeshVisualizerGLBase::setupShaders(GL::Shader& vert, GL::Shader& fra
     /* Extended in MeshVisualizerGL3D for TBN visualization */
     CORRADE_INTERNAL_ASSERT(!(_flags & FlagBase::Wireframe) || _flags & FlagBase::NoGeometryShader || version >= GL::Version::GL320);
     #elif !defined(MAGNUM_TARGET_WEBGL)
-    const GL::Version version = context.supportedVersion({GL::Version::GLES310, GL::Version::GLES300, GL::Version::GLES200});
+    /* ES 3.2 needed for gl_PrimitiveID */
+    const GL::Version version = context.supportedVersion({GL::Version::GLES320, GL::Version::GLES310, GL::Version::GLES300, GL::Version::GLES200});
     /* Extended in MeshVisualizerGL3D for TBN visualization */
     CORRADE_INTERNAL_ASSERT(!(_flags & FlagBase::Wireframe) || _flags & FlagBase::NoGeometryShader || version >= GL::Version::GLES310);
     #else