Replaces the use of gl_generator with the glow wrapper:
* Add glow dependency, based on glow 0.6.1
* Pin version of glow fork for https://github.com/iceiix/glow/pull/1 until #442 renderbuffer
* Remove gl module, steven_gl
Porting details:
* Initialize glow in src/gl/mod.rs
* Call gl methods on glow context
* glow uses camelcase
* Import glow::HasContext trait, finds draw_elements etc.
* Fix mismatched types, glow uses Option and &str instead of raw pointers
* Fix uniform_location, glow already returns Some(u32)
* uniform_location: convert i32 to u32 for Uniform
* Fix attribute_location
* Fix shader creation Result u32 type
* Fix passing GLvoid and 2d/3d
* Fix missing Options type mismatches
* Offsets are i32 in glow, not GLvoid
* Fix clear_buffer using _f32_slice
* Delete methods are singular not plural
* glBufferData -> buffer_data_u8_slice
* buffer_sub_data_u8_slice
* Update more glow method wrapper names found by reviewing glow native platform
* Remove unused multi_draw_elements, can be replaced by draw_elements in a loop and it has no WebGL equivalent
* glow implements glMapBufferRange
* Remove unused read_buffer
* glow's deletes automatically pass 1 and take no reference
* shader_source() accepts &str directly; removes last of std::ptr
* Pass uniform Option<u32>
* Fix bool passing normalized parameter
* Fix draw_buffers parameter
* Stop unnecessarily returning context from gl::init
* Getting shader info is unsafe
* Unwrapping static mut is unsafe
* Use unsafe raw pointers for global mutable context
* Fix initializing GL objects wrappers from glow wrappers
* Unbinding framebuffers uses None
* Uppercase global to fix warning
* Shaders return Some instead of None
* Unbox the context to a raw pointer
* Use tex_image_2d_multisample added in glow fork
* Implement uniform_location, fixing unwrap None failed
* Add tex_sub_image_3d, using PixelUnpackData::Slice
* set_matrix4: transmute the Matrix4 since it is repr(C)
* get_pixels -> get_tex_image -> glGetTexImage, with PixelPackData::Slice
* Wrap sub_image_2d (glTexSubImage2D) and fix warnings
* Implement set_float_multi_raw and set_matrix4_multi, using from_raw_parts
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