MSAA causing render artefacts in Sprites

## Bevy version

0.12

## Operating system & version

Ubuntu 18.04

## What you did

Run the bevy `sprite` example

## What you expected to happen
See a nice Bevy icon.

## What actually happened

## Additional information
I tracked this change down to c2da7800e3671ad92e775529070a814d0bc2f5f8 (this commit added MSAA to sprites). Turning off MSAA with `app.insert_resource(Msaa { samples: 1 })` resolves the issue.

Part of `vulkaninfo` output:
```
GPU0:
VkPhysicalDeviceProperties:
---------------------------
apiVersion = 4202691 (1.2.195)
driverVersion = 88092674 (0x5403002)
vendorID = 0x8086
deviceID = 0x3e9b
deviceType = PHYSICAL_DEVICE_TYPE_INTEGRATED_GPU
deviceName = Intel(R) UHD Graphics 630 (CFL GT2)
```