Ionic Resources splash too large.

When generating a splash with `ionic cordova resources -s` I get the following error:

```
✔ Filtering out image resources that do not need regeneration - done!
✖ Uploading source images to prepare for transformations - failed!
HTTP Error 400: POST https://res.ionic.io/api/v1/upload
{"Error":"image too large: 27528755","Width":0,"Height":0,"Type":"","Vector":false}
```

Changing the resolution any smaller results in an error that it is too small for the devices.

Ionic Info:
```
cli packages: (/Users/redacted/Desktop/Barjs/node_modules)

@ionic/cli-utils : 1.12.0
ionic (Ionic CLI) : 3.12.0

global packages:

cordova (Cordova CLI) : 7.0.1

local packages:

@ionic/app-scripts : 1.2.5
Cordova Platforms : ios 4.4.0
Ionic Framework : ionic-angular 2.3.0

System:

ios-deploy : 1.9.0
ios-sim : 5.0.6
Node : v6.9.2
npm : 3.10.9
OS : macOS Sierra
Xcode : Xcode 9.0 Build version 9A235

Misc:

backend : legacy
```

從 `ionic cordova resources -s` 命令開始，追蹤來源圖像上傳至 `https://res.ionic.io/api/v1/upload` 的流程。使用符合裝置解析度要求的圖像重現 HTTP 400；當有效的啟動畫面圖像不再僅因上傳 payload 過大而失敗時，即表示完成。

由索引模型根據 Issue 內容生成。