gl_shader_decompiler: Implement gl_ViewportIndex and gl_Layer in vertex shaders

This commit implements gl_ViewportIndex and gl_Layer in vertex and
geometry shaders. In the case it's used in a vertex shader, it requires
ARB_shader_viewport_layer_array. This extension is available on AMD and
Nvidia devices (mesa and proprietary drivers), but not available on
Intel on any platform. At the moment of writing this description I don't
know if this is a hardware limitation or a driver limitation.

In the case that ARB_shader_viewport_layer_array is not available,
writes to these registers on a vertex shader are ignored, with the
appropriate logging.

src/video_core/shader/shader_ir.h

@@ -121,6 +121,18 @@ public:
         return static_cast<std::size_t>(coverage_end * sizeof(u64));
     }
 
+    bool UsesLayer() const {
+        return uses_layer;
+    }
+
+    bool UsesViewportIndex() const {
+        return uses_viewport_index;
+    }
+
+    bool UsesPointSize() const {
+        return uses_point_size;
+    }
+
     bool HasPhysicalAttributes() const {
         return uses_physical_attributes;
     }
@@ -343,6 +355,9 @@ private:
     std::set<Image> used_images;
     std::array<bool, Tegra::Engines::Maxwell3D::Regs::NumClipDistances> used_clip_distances{};
     std::map<GlobalMemoryBase, GlobalMemoryUsage> used_global_memory;
+    bool uses_layer{};
+    bool uses_viewport_index{};
+    bool uses_point_size{};
     bool uses_physical_attributes{}; // Shader uses AL2P or physical attribute read/writes
 
     Tegra::Shader::Header header;