render: constified some local variables in SDL_AllocateRenderVertices.

src/render/SDL_render.c

@@ -287,10 +287,10 @@ void *
 SDL_AllocateRenderVertices(SDL_Renderer *renderer, const size_t numbytes, const size_t alignment, size_t *offset)
 {
     const size_t needed = renderer->vertex_data_used + numbytes + alignment;
-    size_t current_offset = renderer->vertex_data_used;
+    const size_t current_offset = renderer->vertex_data_used;
 
-    size_t aligner = (alignment && ((current_offset & (alignment - 1)) != 0)) ? (alignment - (current_offset & (alignment - 1))) : 0;
-    size_t aligned = current_offset + aligner;
+    const size_t aligner = (alignment && ((current_offset & (alignment - 1)) != 0)) ? (alignment - (current_offset & (alignment - 1))) : 0;
+    const size_t aligned = current_offset + aligner;
 
     if (renderer->vertex_data_allocation < needed) {
         const size_t current_allocation = renderer->vertex_data ? renderer->vertex_data_allocation : 1024;