[Fix] (MG_Impl): create sampler objects at Gen; guard combined-format CopyTexImage

glGenSamplers creates the sampler objects themselves (unlike texture and
buffer names), so glIsSampler must answer GL_TRUE before any bind - the
names now get their state vectors at Gen time (KHR-GL33.api.coverage).

glCopyTexImage2D with a combined DEPTH_STENCIL internalformat now
requires both halves in the read framebuffer and reports
GL_INVALID_OPERATION when only the depth or only the stencil attachment
point is populated (packed_depth_stencil.validate_errors.*).

MobileGL/MG_Impl/GLImpl/Sampler/GL_Sampler.cpp

@@ -185,6 +185,11 @@ namespace MobileGL::MG_Impl::GLImpl {
         static thread_local Vector<GLuint> names;
         MG_State::pGLContext->GenSamplerNames(count, names);
         Memcpy(samplers, names.data(), count * sizeof(GLuint));
+        // Unlike textures/buffers, glGenSamplers CREATES the sampler objects: each name
+        // is immediately a sampler (glIsSampler == GL_TRUE before any bind).
+        for (GLsizei i = 0; i < count; ++i) {
+            MG_State::pGLContext->CreateSamplerObject(names[i]);
+        }
     }
 
     void DeleteSamplers_State(GLsizei count, const GLuint* samplers) {