gl: Fix some type conversion warnings with MSVC

MSVC complained about implicit conversion between GstGLFormat* and guint*
8 jobs for fix-gl-warnings in 15 minutes and 54 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #79493
gst indent

00:00:29

passed #79492
manifest

00:00:19

 
  Build
passed #79494
build fedora x86_64

00:03:21

 
  Test
passed #79495
check fedora

00:03:23

 
  Full Builds
passed #79496
build android arm64 api28

00:04:24

passed #79498
build cerbero cross win32

00:08:07

passed #79499
build cerbero cross win64

00:08:17

passed #79497
build cerbero fedora x86_64

00:08:39