Fix assertion in ImGui::NewFrame if getBackbufferSurface returns null.
Fix assertion in ImGui::NewFrame if getBackbufferSurface returns null.

Previously, in cases where the backbuffer was null, we would call layer->onPrePaint (which starts a new frame with ImGui::NewFrame) without a matching call to layer->onPaint (which calls ImGui::Render). This causes the ImGui frame counter to go out of sync, which leads to an assertion when NewFrame is next called.

(The backbuffer can become null if unsupported settings are used, e.g. a MSAA setting above the maximum supported by your GPU.)

Change-Id: I1b8fbaa1bf78ad5e1aa41fd04eda6be206c952b4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/283196
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>

aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia