layers: restructure code generation for layers
layers: restructure code generation for layers

Changed _generate_dispatch_entrypoints in xgl-layer-generate.py
to remove specific layer handling.
Instead pass it a "generate_intercept" function to decide which
API calls to intercept and output layer specific intercept code.
Output layer_intercept_proc() function per layer instead of using
xgl_generic_intercept_proc_helper.h so intercept lists can vary
for different generated layers.

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools