Fix signed vs. unsigned mismatch warnings

EGL header helpfully has EGLBoolean defined as unsigned and enums, including
EGL_TRUE, as signed. Khronos has not found a satisfactory way to amend
the headers, so we'll need to cast the warnings away.

Bug: 33214264
Test: Run EGLTest on Pixel XL
Change-Id: I63240c83a8a4097895d0731c763651fca89cfc55