spirv: Add SpvCapabilityShaderViewportIndexLayerEXT

This capability allows gl_ViewportIndex and gl_Layer to also be used
as outputs in Vertex and Tesselation shaders.

v2: Make conditional to the capability, add gl_Layer, add tesselation
    shaders. (Iago)

v3: Don't export to tesselation control shader.

v4: Add Reviewd-by tag.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/compiler/shader_info.h

@@ -43,6 +43,7 @@ struct spirv_supported_capabilities {
    bool multiview;
    bool variable_pointers;
    bool storage_16bit;
+   bool shader_viewport_index_layer;
 };
 
 typedef struct shader_info {