add maxzoom uniform to support external tilesets

[mollymerp]

ported from mapbox/mapbox-gl-js#6103

still need to update the gl-js sha when the above PR is merged
cc @nickidlugash @ibesora

[ibesora]

Looks good! And btw, it's such a great addition!

[mollymerp]

getting the following crash for the Google Pixel device test on Firebase

*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
Build fingerprint: 'google/sailfish/sailfish:8.0.0/OPR3.170623.008/4294783:user/release-keys'
Revision: '0'
ABI: 'arm'
pid: 9364, tid: 16922, name: GLThread 2360  >>> com.mapbox.mapboxsdk.testapp <<<
signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0xa0
Cause: null pointer dereference
    r0 00000000  r1 00000000  r2 00000001  r3 00000001
    r4 aa20b044  r5 d2a7be28  r6 00000000  r7 d2a7be10
    r8 d2a7be18  r9 1e1a4ae9  sl 000001b3  fp 11e1a300
    ip d5781ad1  sp d2a7b850  lr d55dcb81  pc d55dd190  cpsr 600b0030
backtrace:
    #00 pc 00626190  /data/app/com.mapbox.mapboxsdk.testapp-Xe4Z7LfgLazMY8Tsq7MVfQ==/lib/arm/libmapbox-gl.so
    #01 pc 00625b7d  /data/app/com.mapbox.mapboxsdk.testapp-Xe4Z7LfgLazMY8Tsq7MVfQ==/lib/arm/libmapbox-gl.so
    #02 pc 002cd227  /data/app/com.mapbox.mapboxsdk.testapp-Xe4Z7LfgLazMY8Tsq7MVfQ==/lib/arm/libmapbox-gl.so
    #03 pc 002c0edd  /data/app/com.mapbox.mapboxsdk.testapp-Xe4Z7LfgLazMY8Tsq7MVfQ==/lib/arm/libmapbox-gl.so
    #04 pc 0010ec5d  /data/app/com.mapbox.mapboxsdk.testapp-Xe4Z7LfgLazMY8Tsq7MVfQ==/lib/arm/libmapbox-gl.so
    #05 pc 001168bf  /data/app/com.mapbox.mapboxsdk.testapp-Xe4Z7LfgLazMY8Tsq7MVfQ==/lib/arm/libmapbox-gl.so
    #06 pc 00116855  /data/app/com.mapbox.mapboxsdk.testapp-Xe4Z7LfgLazMY8Tsq7MVfQ==/lib/arm/libmapbox-gl.so
    #07 pc 0011681d  /data/app/com.mapbox.mapboxsdk.testapp-Xe4Z7LfgLazMY8Tsq7MVfQ==/lib/arm/libmapbox-gl.so
    #08 pc 00116949  /data/app/com.mapbox.mapboxsdk.testapp-Xe4Z7LfgLazMY8Tsq7MVfQ==/lib/arm/libmapbox-gl.so
    #09 pc 00116909  /data/app/com.mapbox.mapboxsdk.testapp-Xe4Z7LfgLazMY8Tsq7MVfQ==/lib/arm/libmapbox-gl.so
    #10 pc 003e0269  /system/lib/libart.so (art_quick_generic_jni_trampoline+40)
    #11 pc 00171093  /dev/ashmem/dalvik-jit-code-cache (deleted)

🕵️‍♀️