gl_rasterizer: Fix StartAddress handling with indexed draw calls.

We uploaded the wrong data before. So the offset on the host GPU pointer may work for the first vertices, the last ones run out bounds.
Let's just offset the upload instead.

src/video_core/renderer_opengl/gl_rasterizer.cpp

@@ -484,8 +484,13 @@ void RasterizerOpenGL::DrawArrays() {
     GLintptr index_buffer_offset = 0;
     if (is_indexed) {
         MICROPROFILE_SCOPE(OpenGL_Index);
-        index_buffer_offset =
-            buffer_cache.UploadMemory(regs.index_array.StartAddress(), index_buffer_size);
+
+        // Adjust the index buffer offset so it points to the first desired index.
+        auto index_start = regs.index_array.StartAddress();
+        index_start += static_cast<size_t>(regs.index_array.first) *
+                       static_cast<size_t>(regs.index_array.FormatSizeInBytes());
+
+        index_buffer_offset = buffer_cache.UploadMemory(index_start, index_buffer_size);
     }
 
     SetupShaders();
@@ -499,10 +504,6 @@ void RasterizerOpenGL::DrawArrays() {
     if (is_indexed) {
         const GLint base_vertex{static_cast<GLint>(regs.vb_element_base)};
 
-        // Adjust the index buffer offset so it points to the first desired index.
-        index_buffer_offset += static_cast<GLintptr>(regs.index_array.first) *
-                               static_cast<GLintptr>(regs.index_array.FormatSizeInBytes());
-
         if (gpu.state.current_instance > 0) {
             glDrawElementsInstancedBaseVertexBaseInstance(
                 primitive_mode, regs.index_array.count,