intel: Handle GL_RGB8 for glCopyTex(Sub)Image.

Avoids an unnecessary fallback.

src/mesa/drivers/dri/intel/intel_tex_copy.c

@@ -74,6 +74,7 @@ get_teximage_source(struct intel_context *intel, GLenum internalFormat)
    case GL_RGBA:
    case GL_RGBA8:
    case GL_RGB:
+   case GL_RGB8:
       return intel_readbuf_region(intel);
    default:
       return NULL;