GL: fix out of bounds accesses in Shader limit queries.

Sigh, it really took quite a while (half a decade??) to discover and fix
this.

doc/changelog.dox

@@ -337,6 +337,9 @@ See also:
     accident and it was also not really moving everything properly, especially
     when delayed creation was done on the moved-to object
 -   @ref GL::Renderer::MemoryBarrier::ShaderStorage had an incorrect value
+-   @ref GL::Shader limit queries for a particular shader stage on desktop were
+    out-of-bounds array accesses, causing wrong or random values being returned
+    for most shader-related limits
 -   Fixed assertions related to OpenGL driver workarounds when the proprietary
     AMDGPU PRO drivers are used on Linux
 -   @ref Platform::EmscriptenApplication randomly created antialiased contexts

src/Magnum/GL/Implementation/ShaderState.h

@@ -45,7 +45,7 @@ struct ShaderState {
 
     enum: std::size_t {
         #if !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
-        StageCount = 4
+        StageCount = 6
         #else
         StageCount = 2
         #endif