On systems lacking the EGL_EXT_present_opaque extension, some
compositors treat any buffer with an alpha channel as per-pixel
transparent.
This commit ignores any EGLConfig with an alpha channel if the extension
is missing and the window is created with GLFW_TRANSPARENT_FRAMEBUFFER
set to false.
This is technically not a breaking change since GLFW_ALPHA_BITS is not
a hard constraint, but it is still going to inconvenience anyone using
the framebuffer alpa channel to store other kinds of data.
Related to #1895
(cherry picked from commit ef6c9d8b4f)
8b4f1ebbea
