Port libsdl-org#12581 to SDL2

This fixes undefined behavior resulting from adding offsets to nullptr.

Author: dschuff

src/render/opengles2/SDL_render_gles2.c

@@ -975,8 +975,8 @@ static int SetDrawState(GLES2_RenderData *data, const SDL_RenderCommand *cmd, co
     }
 
     if (texture) {
-        SDL_Vertex *verts = (SDL_Vertex *)(((Uint8 *)vertices) + cmd->data.draw.first);
-        data->glVertexAttribPointer(GLES2_ATTRIBUTE_TEXCOORD, 2, GL_FLOAT, GL_FALSE, stride, (const GLvoid *)&verts->tex_coord);
+        uintptr_t base = (uintptr_t)vertices + cmd->data.draw.first; // address of first vertex, or base offset when using VBOs.
+        data->glVertexAttribPointer(GLES2_ATTRIBUTE_TEXCOORD, 2, GL_FLOAT, GL_FALSE, stride, (const GLvoid *)(base + offsetof(SDL_Vertex, tex_coord)));
     }
 
     if (GLES2_SelectProgram(data, imgsrc, texture ? texture->w : 0, texture ? texture->h : 0) < 0) {
@@ -1009,9 +1009,9 @@ static int SetDrawState(GLES2_RenderData *data, const SDL_RenderCommand *cmd, co
 
     /* all drawing commands use this */
     {
-        SDL_VertexSolid *verts = (SDL_VertexSolid *)(((Uint8 *)vertices) + cmd->data.draw.first);
-        data->glVertexAttribPointer(GLES2_ATTRIBUTE_POSITION, 2, GL_FLOAT, GL_FALSE, stride, (const GLvoid *)&verts->position);
-        data->glVertexAttribPointer(GLES2_ATTRIBUTE_COLOR, 4, GL_UNSIGNED_BYTE, GL_TRUE /* Normalized */, stride, (const GLvoid *)&verts->color);
+        uintptr_t base = (uintptr_t)vertices + cmd->data.draw.first; // address of first vertex, or base offset when using VBOs.
+        data->glVertexAttribPointer(GLES2_ATTRIBUTE_POSITION, 2, GL_FLOAT, GL_FALSE, stride, (const GLvoid *)(base + offsetof(SDL_VertexSolid, position)));
+        data->glVertexAttribPointer(GLES2_ATTRIBUTE_COLOR, 4, GL_UNSIGNED_BYTE, GL_TRUE /* Normalized */, stride, (const GLvoid *)(base + offsetof(SDL_VertexSolid, color)));
     }
 
     return 0;
@@ -1200,7 +1200,8 @@ static int GLES2_RunCommandQueue(SDL_Renderer *renderer, SDL_RenderCommand *cmd,
     if (data->current_vertex_buffer >= SDL_arraysize(data->vertex_buffers)) {
         data->current_vertex_buffer = 0;
     }
-    vertices = NULL; /* attrib pointers will be offsets into the VBO. */
+    // attrib pointers will be offsets into the VBO.
+    vertices = (void *)(uintptr_t)0; // must be the exact value 0, not NULL (the representation of NULL is not guaranteed to be 0).
 #endif
 
     while (cmd) {