[Feat] (MG_Impl, MG_Test): validate the direct-state-access texture copies

CopyTextureSubImage1D and 3D were do-nothing stubs and the 2D form checked only
its effective target, so all 28 conditions in
direct_state_access.textures_copy_errors went unreported: level and region
bounds, and every read-framebuffer precondition.

The read-framebuffer half lands in FramebufferImpl as ValidateReadFramebufferForCopy -
incomplete read framebuffer (INVALID_FRAMEBUFFER_OPERATION), a read buffer that
names no attachment, and a multisampled read buffer (both INVALID_OPERATION). It
decides multisampledness by attachment kind rather than by sample count alone,
because a TEXTURE_2D_MULTISAMPLE attachment sets SAMPLE_BUFFERS even when its
sample count is one - which is exactly what the CTS attaches, and what a
renderbuffer-only check would have missed.

The texture half is ValidateCopyTextureSubImage, shared by all three forms; 1D
and 3D also get the effective-target rule their form specifies.

NOTE: the copy itself is still not implemented for 1D and 3D - CopyTexSubImage1D_State
and CopyTexSubImage3D_State remain TODOs and no backend exposes anything but a
2D blit - so direct_state_access.textures_copy stays red. Only the errors are
complete, which is what un-stubbing these two entry points buys; both carry a
comment saying so.

CopyTextureSubImage2DUsesNamedObjectAndRestoresBinding had been passing a
storage-less texture and no read framebuffer, which the new validation correctly
rejects. It now sets up a legal copy, so it still measures the by-name plumbing
it was written for.

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

MobileGL/MG_Impl/GLImpl/Exporting/Definitions.cpp

@@ -1062,9 +1062,9 @@ DECLARE_GL_FUNCTION_HEAD(void, TextureSubImage3D, GLuint texture, GLint level, G
 DECLARE_GL_FUNCTION_STUB_HEAD(void, CompressedTextureSubImage1D, GLuint texture, GLint level, GLint xoffset, GLsizei width, GLenum format, GLsizei imageSize, const void* data) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, CompressedTextureSubImage1D, texture, level, xoffset, width, format, imageSize, data)
 DECLARE_GL_FUNCTION_STUB_HEAD(void, CompressedTextureSubImage2D, GLuint texture, GLint level, GLint xoffset, GLint yoffset, GLsizei width, GLsizei height, GLenum format, GLsizei imageSize, const void* data) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, CompressedTextureSubImage2D, texture, level, xoffset, yoffset, width, height, format, imageSize, data)
 DECLARE_GL_FUNCTION_STUB_HEAD(void, CompressedTextureSubImage3D, GLuint texture, GLint level, GLint xoffset, GLint yoffset, GLint zoffset, GLsizei width, GLsizei height, GLsizei depth, GLenum format, GLsizei imageSize, const void* data) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, CompressedTextureSubImage3D, texture, level, xoffset, yoffset, zoffset, width, height, depth, format, imageSize, data)
-DECLARE_GL_FUNCTION_STUB_HEAD(void, CopyTextureSubImage1D, GLuint texture, GLint level, GLint xoffset, GLint x, GLint y, GLsizei width) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, CopyTextureSubImage1D, texture, level, xoffset, x, y, width)
+DECLARE_GL_FUNCTION_HEAD(void, CopyTextureSubImage1D, GLuint texture, GLint level, GLint xoffset, GLint x, GLint y, GLsizei width) DECLARE_GL_FUNCTION_END_NO_RETURN(void, CopyTextureSubImage1D, texture, level, xoffset, x, y, width)
 DECLARE_GL_FUNCTION_HEAD(void, CopyTextureSubImage2D, GLuint texture, GLint level, GLint xoffset, GLint yoffset, GLint x, GLint y, GLsizei width, GLsizei height) DECLARE_GL_FUNCTION_END_NO_RETURN(void, CopyTextureSubImage2D, texture, level, xoffset, yoffset, x, y, width, height)
-DECLARE_GL_FUNCTION_STUB_HEAD(void, CopyTextureSubImage3D, GLuint texture, GLint level, GLint xoffset, GLint yoffset, GLint zoffset, GLint x, GLint y, GLsizei width, GLsizei height) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, CopyTextureSubImage3D, texture, level, xoffset, yoffset, zoffset, x, y, width, height)
+DECLARE_GL_FUNCTION_HEAD(void, CopyTextureSubImage3D, GLuint texture, GLint level, GLint xoffset, GLint yoffset, GLint zoffset, GLint x, GLint y, GLsizei width, GLsizei height) DECLARE_GL_FUNCTION_END_NO_RETURN(void, CopyTextureSubImage3D, texture, level, xoffset, yoffset, zoffset, x, y, width, height)
 DECLARE_GL_FUNCTION_HEAD(void, TextureParameterf, GLuint texture, GLenum pname, GLfloat param) DECLARE_GL_FUNCTION_END_NO_RETURN(void, TextureParameterf, texture, pname, param)
 DECLARE_GL_FUNCTION_HEAD(void, TextureParameterfv, GLuint texture, GLenum pname, const GLfloat* param) DECLARE_GL_FUNCTION_END_NO_RETURN(void, TextureParameterfv, texture, pname, param)
 DECLARE_GL_FUNCTION_HEAD(void, TextureParameteri, GLuint texture, GLenum pname, GLint param) DECLARE_GL_FUNCTION_END_NO_RETURN(void, TextureParameteri, texture, pname, param)