[Feat] (MG_Impl): give the by-name texture image queries their error set

glGetTextureImage resolved a texture by name and went straight to the read,
skipping every object-level rule glGetTexImage enforces through
GetTexImage_State - and on DirectVulkan it skipped the level checks in
CopyTextureImageToClientOrPBO_State as well, because that backend answers
GetTextureImage itself. Fifteen of the sixteen conditions in
direct_state_access.textures_image_query_errors went unreported.

The object-level half of that error set now lives in ValidateTextureImageQuery
and both entry points run it. Three rules are new rather than merely relocated:

- Multisample and buffer textures are not in the accepted target list; neither
  has a single image to return.
- The destination-size checks (bufSize, and the span written into a bound pixel
  pack buffer) move ahead of the read. They existed, but downstream of it, where
  any early bail-out - an unmapped level, a pack step that declines the format -
  swallowed them. Both measure the tightly packed span summed over the object's
  faces, which is the least a query can produce, so nothing that would have fit
  is rejected.
- IsDepthLikeInternalFormat had no case for StencilIndex8, so a colour client
  format read back against a stencil-only texture looked like a matching pair.

glGetCompressedTextureImage was a do-nothing stub. It validates the name and the
level, then reports INVALID_OPERATION: no format MobileGL can hold is
compressed, and answering GL_NO_ERROR without writing would hand the caller
stale memory - the same reasoning GetCompressedTexImage_State already follows.

Takes direct_state_access.textures_image_query_errors from failing to passing on
both backends.

MobileGL/MG_Impl/GLImpl/Texture/Validators.cpp

@@ -226,6 +226,9 @@ namespace MobileGL::MG_Impl::GLImpl::TextureImpl {
             case TextureInternalFormat::Depth24Stencil8:
             case TextureInternalFormat::Depth32FStencil8:
             case TextureInternalFormat::DepthStencil:
+            // Stencil-only is not a colour format either: a colour client format read against a
+            // STENCIL_INDEX8 texture has to be the same INVALID_OPERATION as against a depth one.
+            case TextureInternalFormat::StencilIndex8:
                 return true;
             default:
                 return false;