Remove GLES2/WebGL1 support.

[floooh]

CHANGED:

• SOKOL_GLES2 define no longer valid
• Emscripten: link with option -s USE_WEBGL2=1
• sokol_gfx.h:
  • SG_PIXELFORMAT_DEPTH now resolves to GL_DEPTH_COMPONENT32F (consistent with other backends)
  • depth/stencil images are now regular GL textures (not RenderBuffers) - but still some irregularities for MSAA depth textures, e.g. no resolve in sg_end_pass() => those things will be cleaned up in Revamp MSAA images and render passes. #816
  • sg_gl_context_desc removed
  • sg_desc.gl removed
  • in sg_begin_pass() only use glClearBuffer functions
  • in sg_end_pass() MSAA resolve now does glInvalidateFramebuffer() on the MSAA buffer
  • sg_features: instancing, multiple_render_targets, msaa_render_targets, imagetype_3d, imagetype_array removed (these are now all guaranteed across all backends)
  • no more wrapper macros for instanced rendering functions
• sokol_app.h:
  • sapp_desc.gl_force_gles2 removed
  • sapp_gles2() function removed
  • any GLES3-to-GLES2 fallbacks removed

TODO:

• update changelog
• fix glfw samples
• fix emscripten samples
• fix d3d11 samples
• fix Win32 GL loader defines
• fix EGL+GLES code path on Linux
• fix GL context setup on Android
• fix GL context setup on iOS
• check on:
  • macOS
  • iOS
  • Windows
  • Linux (also EGL)
  • Android
  • Web
• change all embedded shaders in utility headers from v100 to v300es:
  • sokol_debugtext.h
  • sokol_fontstash.h
  • sokol_gl.h
  • sokol_imgui.h
  • sokol_nuklear.h
  • sokol_spine.h