godot: GLES 2 fallback not working on Desktop Safari

Godot version: 3.1 rc2 OS/device including version: Latest OS X

Issue description: When compiling dodge the creep for GLES3 and enabling the GLES2 fallback in project settings, the game fails to load in desktop Safari with the following error:

[Warning] exception thrown: TypeError: undefined is not an object (evaluating 'GLctx.getParameter'),_glGetString@file:///Users/bobby/Downloads/godot-demo-projects-master/2d/dodge_the_creeps/Dodge%20the%20Creeps.js:7:264749 (Dodge the Creeps.js, line 7)
wasm-stub@[wasm code]
<?>.wasm-function[__ZN15RasterizerGLES310initializeEv]@[wasm code]
<?>.wasm-function[__ZN18VisualServerRaster4initEv]@[wasm code]
<?>.wasm-function[__ZN13OS_JavaScript10initializeERKN2OS9VideoModeEii]@[wasm code]
<?>.wasm-function[__ZN4Main6setup2Ey]@[wasm code]
<?>.wasm-function[__ZN4Main5setupEPKciPPcb]@[wasm code]
<?>.wasm-function[_main]@[wasm code]
wasm-stub@[wasm code]
jj@[native code]
file:///Users/bobby/Downloads/godot-demo-projects-master/2d/dodge_the_creeps/Dodge%20the%20Creeps.js:7:316260
callMain@file:///Users/bobby/Downloads/godot-demo-projects-master/2d/dodge_the_creeps/Dodge%20the%20Creeps.js:7:319971
synchronousStart@file:///Users/bobby/Downloads/godot-demo-projects-master/2d/dodge_the_creeps/Dodge%20the%20Creeps.js:216:23
apply@[native code]
promiseReactionJob@[native code]
[Error] undefined is not an object (evaluating 'GLctx.getParameter')
	displayFailureNotice (Dodge the Creeps.html:241)
	promiseReactionJob
[Warning] **ERROR**: Condition ' !_start_success ' is true. returned: false (Dodge the Creeps.js, line 7)
[Warning]    At: main/main.cpp:1277:start() - Condition ' !_start_success ' is true. returned: false (Dodge the Creeps.js, line 7)

Steps to reproduce: Run dodge the creeps and enable the GLES2 fallback in project settings.