Linux: update GLEW to 2.2.0, and enable EGL support in GLEW to match wxWidgets

On Linux, wxGTK by default attempts to use EGL if it is available on the
system, rather than GLX.  Unfortunately, the ancient version of GLEW that we
packaged in did not support EGL, and even if it did, the configuration was
not set up to enable EGL.  To solve this, we:

  * upgrade GLEW to version 2.2.0, from upstream GitHub

  * modify the Bambu build process to enforce that we use GLEW from the
    built dependency

  * remove the "extra" even older GLEW that was packaged

  * modify GLEW's CMake configuration to enable EGL support when it is
    available on the system (using the same test as wxWidgets uses to decide
    whether to enable EGL support); if EGL isn't available at compile time,
    both GLEW and wxWidgets will fall back on GLX

Note that you probably will have to blow away your CMakeCache for this to
work correctly -- otherwise, you may end up with the system GLEW, if you
have one installed (which is probably not what you want -- on Ubuntu, the
system GLEW is GLX, not EGL).

CMakeLists.txt

@@ -484,16 +484,7 @@ if (SLIC3R_STATIC AND NOT SLIC3R_STATIC_EXCLUDE_GLEW)
     set(GLEW_VERBOSE ON)
 endif()
 
-find_package(GLEW)
-if (NOT TARGET GLEW::GLEW)
-    message(STATUS "GLEW not found, using bundled version.")
-    add_library(glew STATIC ${LIBDIR}/glew/src/glew.c)
-    set(GLEW_FOUND TRUE)
-    set(GLEW_INCLUDE_DIRS ${LIBDIR}/glew/include/)
-    target_compile_definitions(glew PUBLIC GLEW_STATIC)
-    target_include_directories(glew PUBLIC ${GLEW_INCLUDE_DIRS})
-    add_library(GLEW::GLEW ALIAS glew)
-endif ()
+find_package(GLEW REQUIRED)
 
 find_package(glfw3 REQUIRED)