Complex Curves examples fail on MacBook

This is in continuation of https://github.com/CindyJS/CindyJS/pull/521#issuecomment-259459037. As stated there, the Complex Curves examples currently fail for me in both Firefox and Chrome on macOS Sierra with a MacBook Pro 2015. No messages are printed to console. The webglreport.com report in Chrome reads thus:

Platform:	MacIntel
Browser User Agent:	Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.71 Safari/537.36
Context Name:	webgl
GL Version:	WebGL 1.0 (OpenGL ES 2.0 Chromium)
Shading Language Version:	WebGL GLSL ES 1.0 (OpenGL ES GLSL ES 1.0 Chromium)
Vendor:	WebKit
Renderer:	WebKit WebGL
Unmasked Vendor:	Intel Inc.
Unmasked Renderer:	Intel(R) Iris(TM) Graphics 6100
Antialiasing:	Available
ANGLE:	No
Major Performance Caveat:	No
Vertex Shader
Max Vertex Attributes:	16
Max Vertex Uniform Vectors:	1024
Max Vertex Texture Image Units:	16
Max Varying Vectors:	15
Best float precision:	[-2127, 2127] (23)
Transform Feedback
Coming in WebGL 2
Rasterizer
Aliased Line Width Range:	[1, 7]
Aliased Point Size Range:	[1, 255]
Fragment Shader
Max Fragment Uniform Vectors:	1024
Max Texture Image Units:	16
float/int precision:	highp/highp
Best float precision:	[-2127, 2127] (23)
Framebuffer
Max Color Buffers:	8
RGBA Bits:	[8, 8, 8, 8]
Depth / Stencil Bits:	[24, 8]
Max Render Buffer Size:	16384
Max Viewport Dimensions:	[16384, 16384]
Textures
Max Texture Size:	16384
Max Cube Map Texture Size:	16384
Max Combined Texture Image Units:	16
Max Anisotropy:	16
Uniform Buffers
Coming in WebGL 2
Supported Extensions:
ANGLE_instanced_arrays
EXT_blend_minmax
EXT_disjoint_timer_query
EXT_frag_depth
EXT_shader_texture_lod
EXT_sRGB
EXT_texture_filter_anisotropic
WEBKIT_EXT_texture_filter_anisotropic
OES_element_index_uint
OES_standard_derivatives
OES_texture_float
OES_texture_float_linear
OES_texture_half_float
OES_texture_half_float_linear
OES_vertex_array_object
WEBGL_compressed_texture_s3tc
WEBKIT_WEBGL_compressed_texture_s3tc
WEBGL_debug_renderer_info
WEBGL_debug_shaders
WEBGL_depth_texture
WEBKIT_WEBGL_depth_texture
WEBGL_draw_buffers
WEBGL_lose_context
WEBKIT_WEBGL_lose_context

Hope this helps.

[kranich]

I think I have come up with a workaround. May need a few days to implement it.

[gagern]

If everything else fails, could you just do a trial run: render some known content to some off-screen canvas and see whether the result is consistent with support for reading pixels or not?