feat: disable purging frame buffer when app in backgroud , just exit

[duduWang20]

If UIApplicationDidReceiveMemoryWarningNotification is posted while the app is in the background , calling [GPUImageFramebufferCache purgeAllUnassignedFramebuffers] may cause app crash . Just exit(0) is a better choice .

Here is the crash info - Crashed: com.sunsetlakesoftware.GPUImage.openGLESContextQueue 0 libGPUSupportMercury.dylib 0x19b850f94 gpus_ReturnNotPermittedKillClient 1 AGXGLDriver 0x1a030c1bc (null) 2 libGPUSupportMercury.dylib 0x19b851f44 gpusSubmitDataBuffers 3 AGXGLDriver 0x1a030d77c (null) + 12704 4 IOAccelerator 0x18e2a9d94 IOAccelContextTestResourceSysMem + 80 5 GLEngine 0x1a0a867f0 gleTestObject + 308 6 GLEngine 0x1a0a86684 glTestObjectAPPLE_Exec + 136 7 CoreVideo 0x18e385368 CVPixelBufferOpenGLESTextureBacking::testTexture() + 92 8 CoreVideo 0x18e381c48 CVOpenGLESTextureCache::bufferBackingNotInUse(CVBufferBacking*) + 160 9 CoreVideo 0x18e378034 CVBufferBacking::releaseUsage() + 104 10 CoreVideo 0x18e378430 CVOpenGLESTexture::finalize() + 56 11 CoreFoundation 0x18bccc358 _CFRelease + 216 12 Trans 0x100960c6c 41-[GPUImageFramebuffer destroyFramebuffer]_block_invoke (GPUImageFramebuffer.m:227) 13 Trans 0x100971970 runSynchronouslyOnVideoProcessingQueue (GPUImageOutput.m:30) 14 Trans 0x100960b94 -[GPUImageFramebuffer destroyFramebuffer] (GPUImageFramebuffer.m:238) 15 Trans 0x100960724 -[GPUImageFramebuffer dealloc] (GPUImageFramebuffer.m:112) 16 CoreFoundation 0x18bc44624 -[NSDictionaryM removeAllObjects] + 520 17 Trans 0x100961e48 __58-[GPUImageFramebufferCache purgeAllUnassignedFramebuffers]_block_invoke (GPUImageFramebufferCache.m:154)