Follow-up from "xwayland: clear pixmaps after creation in rootless mode"

The following discussion from !307 (merged) should be addressed:

• @daenzer started a discussion: (+1 comment)

  @imirkin pointed out on IRC #xorg-devel that glDrawBuffer isn't available in GLES, and @mntmn reported this indeed breaks when glamor is using a GLES context. Since there's no other glDrawBuffer call in glamor, this one can probably just be dropped as well?

  Here's the output of running Xwayland with an OpenGL ES 2.0 context after !307 (merged):

  No provider of glDrawBuffer found.  Requires one of:
      Desktop OpenGL 1.0
  (EE) 
  (EE) Backtrace:
  (EE) 0: /usr/bin/Xwayland (0x400000+0x1499d0) [0x5499d0]
  (EE) 1: /usr/bin/Xwayland (0x400000+0x14c888) [0x54c888]
  (EE) 2: /lib64/libc.so.6 (0x7f08c8a2a000+0x3c6b0) [0x7f08c8a666b0]
  (EE) 3: /lib64/libc.so.6 (gsignal+0x145) [0x7f08c8a66625]
  (EE) 4: /lib64/libc.so.6 (abort+0x12b) [0x7f08c8a4f8d9]
  (EE) 5: /lib64/libepoxy.so.0 (0x7f08c8e66000+0x648c2) [0x7f08c8eca8c2]
  (EE) 6: /lib64/libepoxy.so.0 (0x7f08c8e66000+0x64913) [0x7f08c8eca913]
  (EE) 7: /lib64/libepoxy.so.0 (0x7f08c8e66000+0x6b69f) [0x7f08c8ed169f]
  (EE) 8: /usr/bin/Xwayland (0x400000+0x40ec5) [0x440ec5]
  (EE) 9: /usr/bin/Xwayland (0x400000+0x24bf5) [0x424bf5]
  (EE) 10: /usr/bin/Xwayland (0x400000+0xb1d32) [0x4b1d32]
  (EE) 11: /usr/bin/Xwayland (0x400000+0xb2e77) [0x4b2e77]
  (EE) 12: /usr/bin/Xwayland (0x400000+0xb2283) [0x4b2283]
  (EE) 13: /usr/bin/Xwayland (0x400000+0xb27c6) [0x4b27c6]
  (EE) 14: /usr/bin/Xwayland (0x400000+0xb39c1) [0x4b39c1]
  (EE) 15: /usr/bin/Xwayland (0x400000+0x82e6b) [0x482e6b]
  (EE) 16: /usr/bin/Xwayland (0x400000+0x86d44) [0x486d44]
  (EE) 17: /lib64/libc.so.6 (__libc_start_main+0xf3) [0x7f08c8a511a3]
  (EE) 18: /usr/bin/Xwayland (0x400000+0x18f1e) [0x418f1e]
  (EE) 
  (EE) 
  Fatal server error:
  (EE) Caught signal 6 (Aborted). Server aborting
  (EE)