renderer: fix memory leak

Introduced by 8ef0f369, the `psh` plane shader needs to be aborted
even on a successful subpass merge.

Maybe this API should be redesigned. Oh well.

Fixes #130
12 jobs for master in 7 minutes and 34 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Compile
passed aarch64 #593735
docker aarch64

00:00:10

passed linux #593731
docker amd64

00:00:13

passed macos #593736
amd64 catalina

00:00:08

passed scan #593737
docker amd64

00:00:26

passed static #593732
docker amd64

00:00:11

passed win32 #593733
docker amd64

00:00:12

passed win64 #593734
docker amd64

00:00:09

 
  Test
passed gpu #593739
gpu

00:01:42

78.8285%
passed llvmpipe #593738
docker amd64

00:00:33

 
  Sanitize
passed asan #593740
gpu

00:02:01

passed msan #593741
gpu

00:01:36

passed ubsan #593742
gpu

00:01:42