[Fix] (MG_Impl, MG_Backend): let the backend that can honour a layered attachment have it

NamedFramebufferTextureLayer declined every attachment but layer zero, on both
backends. That was right for DirectVulkan, which maps a GL layer onto a Vulkan
array layer with no notion of a 3D depth slice, but wrong for DirectGLES:
SyncAttachmentObject already routes a layered upload target to
glFramebufferTextureLayer with the attachment's layer passed straight through,
and array storage already carries the real layer count into glTexStorage3D. The
one backend that could render to the layer was being told it could not.

The decision now lives in a DynamicBackendParameters flag, so it is the backend
that answers rather than the entry point guessing. DirectGLES sets it when the
driver resolved glFramebufferTextureLayer; DirectVulkan leaves it false until
VkRenderPassManager tells a depth slice from an array layer.

framebuffers_texture_layer_attachment's colour checks now pass on Espryt for 3D,
2D array and 2D multisample array textures - the case still fails there on cube
map arrays, which DirectGLES gives no storage at all, and on the depth and
stencil halves. No case changes on DirectVulkan, which keeps the old behaviour.

MobileGL/MG_Backend/BackendObject.h

@@ -353,6 +353,12 @@ namespace MobileGL {
             // real ES drivers behind DirectGLES both do. Defaults to true so a backend
             // that never sets it keeps the permissive behaviour.
             Bool SupportsDistinctDepthStencilAttachments = true;
+            // Whether attaching a single layer of a 3D or array texture to a framebuffer actually
+            // renders to that layer. DirectGLES hands the layer straight to
+            // glFramebufferTextureLayer, so it does; DirectVulkan maps a GL layer onto a Vulkan
+            // array layer with no notion of a 3D depth slice, so it does not yet. Defaults to false
+            // so a backend that never sets it gets the conservative answer.
+            Bool SupportsPerLayerFramebufferAttachment = false;
             SizeT MaxShaderStorageBlockSize = 128 * 1024 * 1024;
             Uint32 SubgroupSize = 0;
             Uint32 SubgroupSupportedStages = 0;