Remove direct OpenGL(ES) dependency

IrrlichtMt now provides this for us (see last commit)
fixes minetest#12041

README.md

@@ -249,7 +249,7 @@ General options and their default values:
  ENABLE_CURL=ON - Build with cURL; Enables use of online mod repo, public serverlist and remote media fetching via http
  ENABLE_CURSES=ON - Build with (n)curses; Enables a server side terminal (command line option: --terminal)
  ENABLE_GETTEXT=ON - Build with Gettext; Allows using translations
- ENABLE_GLES=OFF - Build for OpenGL ES instead of OpenGL (requires support by IrrlichtMt)
+ ENABLE_GLES=OFF - Enable extra support code for OpenGL ES (requires support by IrrlichtMt)
  ENABLE_LEVELDB=ON - Build with LevelDB; Enables use of LevelDB map backend
  ENABLE_POSTGRESQL=ON - Build with libpq; Enables use of PostgreSQL map backend (PostgreSQL 9.5 or greater recommended)
  ENABLE_REDIS=ON - Build with libhiredis; Enables use of Redis map backend
@@ -259,7 +259,6 @@ General options and their default values:
  ENABLE_PROMETHEUS=OFF - Build with Prometheus metrics exporter (listens on tcp/30000 by default)
  ENABLE_SYSTEM_GMP=ON - Use GMP from system (much faster than bundled mini-gmp)
  ENABLE_SYSTEM_JSONCPP=ON - Use JsonCPP from system
- OPENGL_GL_PREFERENCE=LEGACY - Linux client build only; See CMake Policy CMP0072 for reference
  RUN_IN_PLACE=FALSE - Create a portable install (worlds, settings etc. in current directory)
  USE_GPROF=FALSE - Enable profiling using GProf
  VERSION_EXTRA= - Text to append to version (e.g. VERSION_EXTRA=foobar -> Minetest 0.4.9-foobar)
@@ -300,8 +299,6 @@ Library specific options:
  OPENAL_DLL - Only if building with sound on Windows; path to OpenAL32.dll
  OPENAL_INCLUDE_DIR - Only if building with sound; directory where al.h is located
  OPENAL_LIBRARY - Only if building with sound; path to libopenal.a/libopenal.so/OpenAL32.lib
- OPENGLES2_INCLUDE_DIR - Only if building with GLES; directory that contains gl2.h
- OPENGLES2_LIBRARY - Only if building with GLES; path to libGLESv2.a/libGLESv2.so
  SQLITE3_INCLUDE_DIR - Directory that contains sqlite3.h
  SQLITE3_LIBRARY - Path to libsqlite3.a/libsqlite3.so/sqlite3.lib
  VORBISFILE_LIBRARY - Only if building with sound; path to libvorbisfile.a/libvorbisfile.so/libvorbisfile.dll.a

src/CMakeLists.txt

@@ -98,30 +98,15 @@ if(BUILD_CLIENT AND ENABLE_SOUND)
  endif()
 endif()
 
-
-option(ENABLE_GLES "Use OpenGL ES instead of OpenGL" FALSE)
+# TODO: this should be removed one day, we can enable it unconditionally
+option(ENABLE_GLES "Enable extra support code for OpenGL ES" FALSE)
 mark_as_advanced(ENABLE_GLES)
 
 option(ENABLE_TOUCH "Enable Touchscreen support" FALSE)
 if(ENABLE_TOUCH)
  add_definitions(-DHAVE_TOUCHSCREENGUI)
 endif()
 
-if(BUILD_CLIENT)
- # transitive dependency from Irrlicht (see longer explanation below)
- if(NOT WIN32)
- if(ENABLE_GLES)
- find_package(OpenGLES2 REQUIRED)
- else()
- set(OPENGL_GL_PREFERENCE "LEGACY" CACHE STRING
- "See CMake Policy CMP0072 for reference. GLVND is broken on some nvidia setups")
- set(OpenGL_GL_PREFERENCE ${OPENGL_GL_PREFERENCE})
-
- find_package(OpenGL REQUIRED)
- endif()
- endif()
-endif()
-
 if(BUILD_CLIENT)
  find_package(Freetype REQUIRED)
 endif()
@@ -544,18 +529,6 @@ if(BUILD_CLIENT)
  )
  endif()
 
- if(ENABLE_GLES)
- target_link_libraries(
- ${PROJECT_NAME}
- ${OPENGLES2_LIBRARIES}
- ${EGL_LIBRARIES}
- )
- else()
- target_link_libraries(
- ${PROJECT_NAME}
- ${OPENGL_LIBRARIES}
- )
- endif()
  if(USE_GETTEXT)
  target_link_libraries(
  ${PROJECT_NAME}