emscripten: Fix undefined behavior in opengles2 renderer

(cherry picked from commit 14deef997cf461b40cfd3e340e98622421035022)

src/render/opengles2/SDL_render_gles2.c

@@ -989,8 +989,8 @@ static bool SetDrawState(GLES2_RenderData *data, const SDL_RenderCommand *cmd, c
     }
 
     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->colorspace : SDL_COLORSPACE_SRGB)) {
@@ -1023,9 +1023,9 @@ static bool SetDrawState(GLES2_RenderData *data, const SDL_RenderCommand *cmd, c
 
     // 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_FLOAT, 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_FLOAT, GL_TRUE /* Normalized */, stride, (const GLvoid *)(base + offsetof(SDL_VertexSolid, color)));
     }
 
     return true;
@@ -1332,7 +1332,8 @@ static bool 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) {